THE release of a transmitter substance by the nerve impulse depends on the presence of calcium, rather than sodium, on the outside of the axon membrane. There is increasing evidence that inward movement of calcium is one of the first steps in the “electro-secretory” process. This view has been strengthened by recent experiments with tetrodotoxin^1–3, which eliminates the regenerative influx of sodium ions but does not interfere with the release of the transmitter by locally imposed depolarization provided calcium ions are present in the external solution.