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1. The ionic requirements of the somatic and axonal action potentials of `H' neurones of the snail Cryptomphallus aspersa were studied using intracellular micro-electrodes.

2. The overshoot of the somatic action potential increased by 10 mV for a tenfold increase in [Ca²⁺]_o. In calcium-free media the action potential decreased gradually to values of 50 to 90% of the control and they could be completely eliminated with 2 mM-EGTA. The maximum rate of rise also varied with [Ca²⁺]_o.

3. After 2 hr in sodium-free solution the somatic action potential decreased 6% in overshoot and 24% in rate of rise.

4. The somatic action potential was not affected by TTX, 5 x 10^-6 g/ml. Procaine, 18 mM, reduced its rate of rise but did not eliminate it whereas 30 mM-CoCl₂ did.

5. The size of the axonal action potential increased with increased [Na⁺]_o, but decreased with an increase in [Ca²⁺]_o.

6. Procaine, 18 mM, abolished the axonal action potential whereas it was not affected by TTX, 5 x 10^-6 g/ml., nor, usually, by 30 mM-CoCl₂.

7. The results obtained by studying the compound action potential of the nerves were similar to those from axonal action potentials.

8. The possibility that the somatic action potential is mainly calcium dependent while the axonal action potential is mainly produced by sodium is discussed.