Two types of neurons were differentiated, on the basis of electrophysiological properties, in the substantia nigra pars compacta of the guinea pig maintained in slices. While the majority of neurons, presumably dopaminergic neurons, produced a broad spike accompanied by a relatively long-lasting after-hyperpolarization, a small number of neurons were characterized by the generation of a depolarizing after-potential (DAP) following a fast spike. The DAP was depressed by a slight depolarizing shift of the resting membrane potential and by the removal of Ca2+ from the bathing medium, which suggested that it was a low-threshold Ca2+-dependent response. Intracellular staining revealed that the neurons producing DAP were smaller than the major neurons.