Effects of an aromatic tripeptide, L-Lys-L-Phe-L-Tyr and its related dipeptides on the excitability of two identifiable giant neurones (the TAN, tonically autoactive neurone and the PON, periodically oscillating neurone) in the suboesophageal ganglia of the African giant snail, Achatina fulica Férussac, were examined. Of 33 substances examined, three aromatic oligopeptides showed a marked inhibitory effect on the TAN excitability. The critical concentrations of these substances in bath application were: L-Lys-LPhe-L-Tyr (6.6 x 10(-6) approximately 2.2 x 10(-5) M), L-Phe-L-Tyr (9.1 x 10(-6) approximately 3.0 x 10(-5) M) and L-Phe-L-Trp (2.4 x 10(-5) approximately 7.3 x 10(-5) M). The microdrop application (100 micron in diameter) of a solution of the three inhibitory oligopeptides on the TAN surface hyperpolarized the TAN membrane potential and eliminated the spike discharges, in both the physiological and the chloride-free (replaced with acetate) conditions. The current-voltage relationships (I-V curve) of the TAN neuromembrane were measured by the injection of a transmembrane triangular current. The TAN I-V curve measured in the presence of L-Lys-L-Phe-L-Tyr or L-Phe-L-Trp was almost concordant to that of the normal state, if the two I-V curves were superimposed using the firing level as the reference point. The same three aromatic oligopeptides also showed an inhibitory effect on the PON excitability. The critical concentrations needed to produce the effects were much higher than those in the case of the TAN.