1. Spikes in molluscan bursting cells are followed by depolarizing afterpotentials (DAPs) which are not seen in nonbursting cells in the same ganglia. DAPs from successive spikes sum to provide a depolarizing drive capable of sustaining multiple discharge. 2. Subthreshold depolarization activates a DAP-like process in bursters. 3. DAP amplitude increases as the cell is hyperpolarized beyond the potassium equilibrium potential. The amplitude is not changed by intracellular iontophoresis of TEA-Cl. DAP amplitude is reduced by 39% after a 10-min exposure to CA++-free saline, and by 66% after 10 min in 4% Na+ (Tris substituted) saline. The amplitude is unchanged by exposure to K+-free saline. 4. During repetitive stimulation the amplitude of the summed DAP declines. 5. It is concluded that DAP results from a slowly decaying component of Ca++ and Na+ permeability.