1. Mytilus small cardioactive peptide (SCP) was originally isolated from the anterior byssus retractor muscle of Mytilus edulis, with a primary structure, APNFLAYPRLamide. The mechanisms of action of this peptide were examined on identified central neurones of the snail, Helix aspersa, using intracellular recordings and a two electrode voltage clamp. 2. 50 microM APNFLAYPRLamide could elicit a long term excitation on F2 neurones, in normal saline, Na(+)-free saline, Ca(2+)-free saline or in 10 mM Co2+ saline. This indicates that the excitatory effect of APNFLAYPRLamide involved an increase in membrane conductance to both Na+ and Ca2+. 3. 10 microM APNFLAYPRLamide potentiated the Ca2+ inward current of F2 neurones while reversibly reducing the ACh-induced membrane current of these neurones. Forskolin had identical effects to those of APNFLAYPRLamide. This indicates that the action of APNFLAYPRLamide may be through a second messenger. In contrast APNFLAYPRLamide potentiated a cholinergic EPSP evoked in F2 neurones. 4. YPRLamide was inactive even at higher concentrations, for example, 100 microM, while LAYPRLamide exhibited activity but with a lower potency. This indicates that LAYPRLamide is the minimum structure required for activating this class of SCP-like peptide receptor. However, the mode of action of APNFLAYPRLamide on the Na+ current of F2 neurones requires further investigation.