1. The exchange of sodium ions in the cockroach central nervous system has been studied by following the escape of 24Na from isolated abdominal nerve cords, single connectives and ganglia. Particular attention was paid to the initial rapid exchanges of sodium.

2. The escape of sodium ions occurred as a two-stage process, an initial rapid phase eventually giving way to a slower exponential phase of sodium loss. The fast phase of efflux was not affected by the presence of 2:4-dinitrophenol, although this poison significantly reduced the second slow phase of sodium extrusion.

3. The initial fast phase is attributed to a rapid diffusion from an extracellular space, demonstrated by 14C-inulin; the second phase is identified as the slower extrusion from the cellular components of the central nervous system.

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