In identified neurones of the snail Helix aspersa , the Ca 2+ current can be modulated by biogenic amines and peptides in different ways. 1. A reversible increase of Ca 2+ current is evoked by 1 μmoll −1 serotonin (5-HT) in a group ofidentified snail ventral neurones (D6, D7, E1). This 5-HT-induced enhancement of Ca 2+ current is probably mediaed by cyclic GMP. Neither cyclic AMP nor forskolin mimic the effect of 5-HT, but both the intracellular injection of cyclic GMP and the application of zaprinast (an inhibitor of a cyclic-GMP-dependent phosphodiesterase) induce a Ca 2+ current increase. Moreover, when a maximal Ca 2+ current increase is induced by cyclic GMP, 5-HT becomes ineffective and vice versa . 2. Decreases in the Ca 2+ current can be generated by two mechanisms, (a) A reversible decrease of the Ca 2+ current is evoked by both dopamine (1 μmoll −1 ) and the neuropeptide FMRFamide (1 μmoll −1 ) on different identified neurones. Intracellular injections of either cyclic AMP or cyclic GMP do not mimic the effect of dopamine or FMRFamide. Moreover, the intracellular injection of EGTA does not affect the Ca 2+ current decrease induced by these transmitters, (b) An ‘irreversible’ decrease of the Ca 2+ current of the D2 neurone is elicited by 5–20μmoll −1 cholecystokinin octapeptide (CCK8). Intracellular Ca 2+ plays a key role in the intracellular mediation of this effect since the intracellular injection of EGTA prevents the CCK 8-induced decrease of the Ca 2+ current.