Among several identifiable neurones of Achatina fulica Férussac, RAPN (right anterior pallial neurone) was sensitive to the two Mytilus inhibitory peptides (MIPs), H-Gly-Ser-Pro-Met-Phe-Val-NH2 ([Ser2]MIP) and H-Gly-Ala-Pro-Met-Phe-Val-NH2 ([Ala2]MIP), and their fragments, H-Pro-Met-Phe-Val-NH2 (MIP-(3-6)) and H-Met-Phe-Val-NH2 (MIP-(4-6)). These peptides, applied either locally by pneumatic pressure or in the bath, produced a slow outward current with an increase in membrane conductance. The other two related peptides, H-Gly-Ala-Pro-Met-Val-Phe-NH2 ([Ala2, Val5, Phe6]MIP-NH2) and H-Gly-Ala-Pro-Met-NH2 ([Ala2]MIP-(1-4)-NH2), were ineffective. The potency order of the four effective peptides was determined from their dose-response relations: [Ser2]MIP greater than [Ala2]MIP greater than MIP-(3-6) greater than MIP-(4-6). The ED50 of [Ser2]MIP was about 3 X 10(-5) M. Relations between the outward current (nA) and the conductance increase (microseconds) produced by the four peptides were quite linear (Y = 0.03416 X-0.01083, r = 0.98677). The reversal potentials for [Ser2]MIP (ES-MIP) measured with various extracellular K+ concentrations ([K+]0) were fitted to the Nernst equation, being identical with EK. ES-MIP was not affected by changing [Na+]0 and [Cl-]0.