This paper reviews our work on the modulation of voltage-dependent Ca currents in identified snail neurons. Ca currents of snail neurones are enhanced or decreased by neurotransmitters. Serotonin and acetylcholine enhance the Ca current of identified neurons, the effect of serotonin being mediated by cGMP and cGMP-dependent protein kinase. Cholecystokinin (CCK8) and dopamine both decrease the Ca current of identified neurons. The effect of CCK8 is irreversible and involves the activation of protein kinase C. The dopamine-induced decrease in Ca current is reversible and involves an alpha 40 subunit of a snail G protein immunologically and functionally related to alpha o of mammalian brain.