Immunocytochemical methods were used to map the distribution of proctolinlike immunoreactivity in the stomatogastric nervous systems (stomatogastric ganglion (STG), paired commissural ganglia (CG), oesophageal ganglion (OG), and connecting nerves) of three crustacean species: Panulirus interruptus, Cancer borealis, and Homarus americanus. Although the patterns of proctolinlike staining were similar among the three species, some differences were also observed. Over 70% of the proctolinlike material in STGs, as measured by radioimmunoassay, was indistinguishable from authentic proctolin in reverse-phase high-performance liquid chromatography. Bath application of proctolin to STGs from Cancer and Panulirus induced characteristic and robust (though somewhat different) changes in their motor patterns. The threshold concentration was approximately 10(-9)M proctolin, and the effects were dose-dependent. These data suggest that the neuropeptide proctolin serves as a neuromodulator of the stomatogastric ganglion.