FMRFamide (i.e. Phe-Met-Arg-Phe-NH2) application to the C2 neurone of Helix caused a depolarizing response which consisted of a large, rapidly developing, and rapidly desensitizing inward current, underlain by a smaller, slower inward current which did not desensitize. Both currents were carried through sodium-selective channels which were insensitive to D-tubocurarine, and the to the fast sodium channel blockers tetrodotoxin (TTX) and lignocaine. Only the faster, desensitizing current could be blocked by amiloride. FMRFamide also activated two types of unitary inward currents with slightly differing amplitudes in outside-out patches taken from the C2 neurone, both through sodium-selective ion channels. Only the smaller unitary currents readily desensitized and were susceptible to block by amiloride, and they also activated more rapidly. Unitary currents of both types were recorded in outside-out patches in the absence of freely diffusible intracellular mediators, and were also activated when guanosine 5'-O-(2-thiodiphosphate) (GDP [beta-S]) was included in the recording pipette solution. This supports a tight receptor/channel coupling for both responses, with no involvement of GTP-binding proteins. Further, the very fast rate of activation of the smaller channels, which generally carry the major part of the FMRFamide-induced current, strongly indicates that these channels are ligand gated.