Octopamine (OA) levels in each ganglion of the terrestrial snail, Helix pomatia, and the pond snail, Lymnaea stagnalis, were measured by using the HPLC technique. In both species an inhomogeneous distribution of OA was found in the central nervous system. The buccal ganglia contained a concentration of OA (12.6 pmol mg-1 and 18.8 pmol mg-1) that was two to three times higher than the pedal (4.93 pmol mg-1 and 9.2 pmol mg-1) or cerebral (4.46 pmol mg-1 and 4.9 pmol mg-1) ganglia of Helix and Lymnaea, respectively, whereas no detectable amount of OA could be assayed in the visceroparietal complex. In Lymnaea ganglia, the OA uptake into the synaptosomal fraction had a high (Km1 = 4.07 ± 0.51 νM, Vmax1 = 0.56 ±0.11 pmol mg-1 per 20 min), and a low (Km2 = 47.6 ± 5.2 νM, Vmax2 = 4.2 ± 0.27 pmol mg201 per 20 min), affinity component. A specific and dissociable 3H–OA binding to the membrane pellet prepared from the CNS of both Helix and Lymnaea was demonstrated. The Scatchard analysis of the ligand binding data showed a one'binding site, representing a single receptor site. The Kd and Bmax values were found to be 33.7 ± 5.95 nM and 1678 ± 179 fmol g-1 tissue in Helix and 84.9 + 17.4 nM and 3803 ± 515 fmol g-1 tissue in Lymnaea preparation. The pharmacological properties of the putative molluscan OA receptor were characterized in both species and it was demonstrated that the receptor resembled the insect OA2 rather than to the cloned Lymnaea OA receptor. Immunocytochemical labelling demonstrated the presence of OA–immunoreactive neurons and fibres in the buccal, cerebral and pedal ganglia in the central nervous system of both species investigated. Electrophysiological experiments also suggested that the Lymnaea brain possessed specific receptors for OA. Local application of OA onto the identified buccal B2 neuron evoked a hyperpolarization which could selectively be inhibited by the OAergic agents phentolamine, demethylchlordimeform and 2–chloro–4–methyl–2–(phenylimino)–imidazolidine. Among the dopamine antagonists, ergotamine reversibly inhibited the OA response, whereas sulpiride had no effect. Based on our findings, a neurotransmitter–modulator role of OA is suggested in the gastropod CNS.