S-100 proteins are a versatile group of calcium-binding proteins which exert intracellular as well as extracellular functions in a variety of cells. Our immunocytochemical studies, using the central nervous system of the terrestrial snail, Helix pomatia, showed that S-100-like material is present exclusively in a restricted number of neurons distributed in the cerebral ganglia, the left parietal ganglion, and the visceral ganglion. Electrophysiological investigations showed that immunostained neurons spontaneously discharge action potentials in a beating or bursting mode. Voltage clamp experiments further revealed a pronounced N-shaped current-voltage relationship of outward currents, indicating a prominent calcium-activated potassium current in these cells. Extracellular application of the disulfidic form of S-100(s-s) results in suppression of spontaneous discharge activity accompanied by an increase of membrane conductance. The total outward current as well as the inward rectifier current were increased. After removal of the calcium-dependent outward current component, S-100 effects were abolished indicating that S-100 acts on the calcium-activated current component. Our results suggest that S-100-like proteins modulate electrical activity of neurons in the central nervous system of Helix pomatia.