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./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.