The effect of the small cardioactive peptide B (SCP B) on adenylate cyclase activity of the central nervous system (CNS) and some peripheral organs of the freshwater snail Planorbarius corneus was investigated. This peptide stimulates enzyme activity in a dose-dependent manner in all the tested ganglia, in the salivary glands, the buccal mass and the oesophagus. The amount required for half-maximal stimulation is approx 0.1 μM. Moreover, the response of adenylate cyclase to SCP B progressively increases with time, without desensitizing. On the other hand, the peptide is ineffective on heart adenylate cyclase. The responsiveness of adenylate cyclase activity to the peptide and the wide distribution of SCP B-related immunoreactivity in all the tested ganglia suggest that this substance may exert an important role as neurotransmitter as well as neuromodulator in the CNS of the snail. Since SCP B is consistently present in the buccal and cerebral ganglia, associated nerves, salivary glands, buccal mass and oesophagus, and in the light of the appreciable effects exerted by SCP B on adenylate cyclase activity in the examined peripheral tissues, we suggest that the peptide is strongly involved in the control of feeding behaviour of P. corneus.