The neuropeptide designated SCPB (small cardioactive peptide B), the sequence of which has recently been determined, was found in the accessory radula closer muscle, a muscle involved in biting movements. The ganglia and nerves that innervate the accessory radula closer muscle also contain SCPB. At nanomolar concentrations, it enhances the contractions of the muscle. The effect of SCPB on the muscle resembles the effect of an identified serotonergic neuron that previously was shown to mediate behavioral effects that reflect a food arousal state in Aplysia. Like serotonin, SCPB enhances contractions by a postsynaptic action, which appears to involve an increase in cAMP levels in the muscle. Our findings suggest that parallel peptidergic and serotonergic pathways may mediate similar aspects of arousal in Aplysia.