Membrane properties of molluscan neural somata and crustacean axons have been studied by voltage clamp analysis. These neurons generate rhythmic discharge in response to constant current stimulation, and in certain of the molluscan neurons the discharge is spontaneous. Atransiently activated potassium conductance is prominent in both of these types of neurons. Computer simulations have been run in which this transient conductance system was incorporated into modified versions of the Hodgkin-Huxley equations. Repetitive discharge is generated that is very similar to the response of the preparation from which the simulation parameters were derived. Rhythmic, spontaneous discharge is generated on slight modification of the conductance parameters.