1. Transient stimulation of an afferent input to the bag cell neurons of Aplysia californica triggers a 30-min period of spontaneous firing termed the afterdischarge. Measurement of free calcium ion concentrations using calcium-sensitive electrodes revealed a biphasic pattern of elevation of intracellular calcium levels during the afterdischarge. Basal calcium levels at the soma were found to rise rapidly during afferent stimulation and then to decline before the onset of spontaneous firing. This early peak in intracellular calcium was followed by a slower, transient elevation of calcium levels during the period of rapid firing that occurs in the first few minutes of afterdischarge. Stimulation of clusters of bag cell neurons in a calcium-free external medium failed to trigger afterdischarge and produced no changes in basal intracellular calcium levels. 2. When calcium ions in the external medium were replaced by barium ions, stimulation of clusters of bag cell neurons triggered afterdischarges that were characterized by long-duration action potentials. Intracellular calcium levels during these afterdischarges rose slowly over the first few minutes of spontaneous firing. Because calcium-sensitive microelectrodes do not respond to barium ions, these data suggest that stimulation of afterdischarge triggers calcium release from an intracellular compartment. 3. During afterdischarges in barium-containing external media, each broadened action potential produced a discrete transient elevation of intracellular calcium levels. A similar effect was observed in isolated bag cell neurons in primary culture when action potentials were stimulated by depolarizing current pulses in a barium-containing medium. These data suggest that, under these conditions, individual action potentials trigger the release of intracellular calcium from some intracellular pool.