these results are perhaps rather unexpected in view of the obvious difference in shape between these two structures . measurements showed that the surface &amp;sol; volume ratio of the connectives was about 3.5 times greater than that of the relatively massive terminal abdominal ganglion . the point of contrast between the effluxes from the terminal ganglion and from the whole nerve cord used in the previous investigation was the apparent absence , in the case of the isolated ganglion , of a final slow phase of sodium loss in a region of low radioactivity . in the previous study ( Treherne , 1961b ) this phase was tentatively identified with the breakdown of the normal sodium extrusion mechanism in the isolated nerve cord when separated from its tracheal supply . thus according to this hypothesis it could be postulated that in the present experiments the isolation of the ganglion resulted in a less serious interference with the normal metabolism so that the breakdown of sodium extrusion did not occur until later at a very low level of activity beyond the limits of this technique . the present results have shown that , as in the whole abdominal nerve cord ( Treherne , 1961b ) , the rate of loss of sodium was apparently an active process which was slowed down by the presence of 2 : 4-dinitrophenol at relatively low concentration . similarly the extrusion of sodium in the terminal ganglion was reduced in the potassium-free solution , demonstrating a linkage of potassium influx with sodium efflux . the rate of efflux of sodium ions from the terminal abdominal ganglion was not significantly affected by the removal of about 50 % of the connective tissue and cellular sheath . on the basis of these results it must be concluded , therefore , that the rate-limiting process in the efflux of sodium measured by this technique was not the transfer of ions across the cellular perineurium . in addition it follows from this that the diffusion of sodium ions through the connective tissue sheath must also have occurred relatively rapidly , a result which had been previously predicted ( Treherne , 1961a ; Wigglesworth , 1960 ) . the rate-limiting process measured in these experiments must , therefore , be associated with some components of the central nervous system lying at a deeper level than the perineurium . perhaps the most obvious possibility is that the efflux of 24Na measured in these experiments was , in fact , the result of the transfer of sodium ions across the cell membranes of the underlying tissues . in this case the similarity of the t0.5 between the connectives and the terminal ganglion becomes explicable , for under these circumstances the efflux might be expected to be independent of the surface &amp;sol; volume ratio of the whole organ . the results described above do not , of course , give any definite information about the nature of the processes involved in the passage of ions across the perineurium . however , the fact that the presence of dinitrophenol and potassium-free solution appeared to have slightly less effect on sodium efflux in the desheathed preparations might suggest that this layer of cells perhaps plays more than a passive role in the ionic regulation of the central nervous system of this insect . the addition of poison to , or the omission of potassium ions from , the external solution has been shown to produce a fairly rapid slowing down of sodium extrusion from the abdominal nerve cord . the fact that the rate-limiting process is not , apparently , the penetration of the superficial perilemma implies that these changes in the chemical composition of the bathing solution are quickly transmitted to the deeper layers of the central nervous system . this conclusion is perhaps rather unexpected in view of the appreciable delay in the breakdown of normal electrical activity obtained when the insect nervous system was exposed to solutions of high potassium concentration ( Hoyle , 1953 ; Twarog &amp; Roeder , 1956 ) . in some previously published accounts on the entry of 42K and 24Na into the intact abdominal nerve cord of Periplaneta ( Treherne , 1961a , c ) an attempt was made to calculate the fluxes of these ions between the haemolymph and the central nervous system . these ionic movements were calculated with the conventional equations used to describe fluxes in cells and tissues . this procedure involved the assumption that the rate-limiting process was the transfer across the superficial boundary and that the movements within the underlying layers occurred rapidly so that the labelled ions were effectively well mixed . the present results have shown that these assumptions represented an oversimplification and consequently the calculated values have little significance . it is hoped that in a future investigation the fluxes taking place between the central nervous system and the haemolymph can be calculated for this more complex system . summary . 1 . the rate of loss of 24Na from the terminal abdominal ganglion of Periplaneta americana L has been studied by measuring the decline in radioactivity associated with an isolated preparation maintained in flowing physiological solution . 2 . the rate of sodium efflux was substantially reduced in the presence of 0.2 mM &amp;sol; l dinitrophenol and in potassium-free solution . 3 . the extrusion of 24Na was not significantly affected by the removal of the fibrous and cellular sheath surrounding the ganglion . the rate-limiting process in the efflux of sodium measured in the experiments was not , therefore , the transfer of ions across the nerve sheath , but an extrusion from tissues lying at a deeper level in the central nervous system . the kinetics of sodium transfer in the central nervous system of the cockroach , periplaneta americana L . by J E Treherne . A.R.C unit of insect physiology , department of zoology , University of Cambridge . ( received 15 June 1961 ) . introduction . some previous investigations have shown that the exchanges of sodium and potassium ions between the haemolymph and the cockroach central nervous system occurred relatively rapidly ( Treherne , 1961a ) and appeared to be effected by a mechanism involving an active extrusion of sodium ions ( Treherne , 1961b ) . more recently it has also been shown that the measured efflux of sodium ions was not significantly affected by the removal of substantial portions of the cellular and fibrous nerve sheath ( Treherne , 1961c ) . it was concluded from this that the rate-limiting factor measured in these experiments was not the transfer of ions across the perilemma but the extrusion of sodium from the underlying tissues of the central nervous system . thus any rate-limiting movements of ions across the perilemma occurred too rapidly to be measured by the techniques used in the previous investigations . in the present experiments , therefore , an attempt has been made to measure the rapid component of 24Na exchange by determining the rate of loss of radioactivity obtained on washing isolated nerve cords and single connectives and ganglia for relatively short periods in successive volumes of physiological solution . methods . the experiments described in this paper were carried out using the abdominal nerve cords of adult male Periplaneta americana L . in these experiments the nerve cords were made radioactive by soaking them for varying periods in a solution containing 24Na ( 0.1-0.5 mc &amp;sol; ml ) . with short loading periods ( 20 sec - 5.0 min ) the isolated ligatured nerve cords were soaked in the oxygenated physiological solution ; for longer loading periods ( 5-20 min ) the nerve cords of decapitated individuals were perfused with the radioactive solution as described in a previous paper ( Treherne , 1961c ) . the ligatures were tied with threads pulled from 15 denier nylon stockings . the composition of the radioactive solution used was that given by Treherne ( 1961a ) . on removal from the radioactive solution the ligatured nerve cords were carefully blotted and then washed for varying periods in successive 0.2 ml amounts of inactive solution of the same composition . the amount of 24Na remaining in the nerve cord at varying times was determined from the measured radioactivity of the washings . the radioactivity measurements were made with a Mullard MX 123 G.M tube linked to a 100 c Panax scaler . some preliminary measurements were made to estimate the extent of any inulin space in the central nervous system . this was done by soaking ligatured isolated nerve cords for 1 hr in a 3.0 % solution of 14C-labelled inulin ( 3.0 mc &amp;sol; g ) made up in physiological solution . the nerve cords were then washed for 25 sec and the 14C-inulin was extracted by soaking them for 24 hr in the physiological solution . the washing time of 25 sec used was found to be the minimum period necessary to remove 97 % of the radioactivity from the surface of a nerve cord exposed to 14C-inulin for 1 sec . these values are thus likely to be minimum estimates of the inulin space of this organ for some radioactivity must have leaked from within the nerve cord during the washing procedure . in a limited number of cases the rate of loss of 14C-labelled inulin was determined by washing the ligatured isolated nerve cords in successive volumes of the physiological solution as for the 24Na efflux experiments . results . the results illustrated in fig 1 show the decline in radioactivity of some isolated abdominal nerve cords , previously soaked in the solution containing 24Na , when maintained in an inactive solution of the same composition . in all cases semi-logarithmic plots of the results for varying loading times appeared to follow a complex course initially , eventually assuming an exponential form after a period of between 160-200 sec . it was found possible to separate a fast component from the curves for the loss of 24Na from the nerve cords by subtraction from the initial values lying above the line extrapolated to zero time . the separation of an efflux curve into fast and slow components with data plotted semi-logarithmically with respect to time is shown in fig 2 . the fast component illustrated in fig 2 was complex initially , but assumed after a few seconds a simple exponential form with a half-time ( t0.5 ) of approximately 33.0 sec . the half-time for the slow component was , in this case , 260 sec . the escape of 24Na from the isolated abdominal nerve cords was also measured in the presence of 0.5 mM &amp;sol; l 2 : 4-dinitrophenol . the poison was added to the physiological solution during the initial loading period with the 24Na and was present at the same concentration in the inactive solution during the subsequent efflux experiments . previous results ( Treherne , 1961 b ) have shown that there was a slight delay period of a few minutes before the poison affected the rate of extrusion of sodium from the nerve cords . in the present experiments , therefore , the nerve cords which were loaded with 24Na for only short periods ( less than 5 min ) were pretreated with 0.5 mM &amp;sol; l dinitrophenol to maintain a constant exposure to the poison of 5 min before the efflux experiments were commenced . fig 3 shows the escape of 24Na from a poisoned preparation loaded with 24Na for 10 min . in this experiment the fast component was not abolished by the presence of the poison , in fact t0.5 in this case was 33.0 sec , which was the same as that for the normal preparation illustrated in fig 2 . in this particular experiment the slow component for 24Na efflux was , however , much reduced as compared with the normal preparation . the effects of 0.5 mM &amp;sol; l 2 : 4-dinitrophenol on the escape of 24Na from the isolated nerve cords are summarized in table 1 . the results clearly indicate that the presence of the poison affected the slow phase of sodium loss but not the initial fast component . the total activity of the 24Na in the slowly exchanging fraction was estimated by extrapolation of the slow component to zero time . fig 4 illustrates the estimated radioactivity of the slowly escaping fraction at varying times after exposure to the solution containing 24Na . these data would appear to show that the poison had little effect on the rate of accumulation of the radioactive ions in the slowly exchanging fraction . the results are , however , too few to judge the equilibrium level of radioactivity as between the normal and poisoned preparations . the escape of 24Na from isolated ligatured fragments of the central nervous system was studied in some experiments . the loss of radio-sodium from the terminal abdominal ganglion and from the connective between the fourth and fifth abdominal ganglia was found to occur as a two-stage process as for the whole abdominal nerve cord . 