1. The rate of loss of sodium ions from the abdominal nerve cord of Periplaneta has been determined by following the decline in radioactivity of 24Na-loaded nerve cords isolated in flowing Ringer solution.
2. In all of the experiments there was an initial rapid exponential decline in radioactivity which eventually gave way to a second slower phase.
3. The initial exponential extrusion of sodium ions was appreciably reduced by the presence of potassium cyanide and 2:4-dinitrophenol.
4. The rate of sodium efflux was not reduced in sodium-free solutions, but was decreased in the absence of external potassium ions.
5. It is concluded that sodium ions are extruded from the nerve cord by a metabolically maintained secretory mechanism which is also associated with the uptake of potassium ions.