We studied G proteins and regulation of adenylate cyclase in nervous tissue and muscle of Aplysia using bacterial toxin-catalyzed ADP-ribosylation. We identified Gs alpha, a Mr 45,000 cholera toxin substrate, Go alpha, a Mr 40,000 pertussis toxin substrate, and G beta (Mr 37,000) by Western blot analysis with antisera specific for bovine brain G protein subunits. Partial proteolysis suggests that the neuronal pertussis toxin substrates are heterogeneous. The concentration of these substrates in membranes from Aplysia ganglia is similar to that of rat, squid and Helix; in Aplysia nervous tissue, G protein subunits are most enriched in synaptosomes and neuropil. The stimulation of adenylate cyclase by serotonin (5-HT), low concentrations of GTP-gamma-S, and cholera toxin, and the inhibition by high concentrations of GTP-gamma-S that is blocked by pertussis toxin indicate that both a Gs and a Gi protein regulate the Aplysia enzyme. These results support the idea that G proteins in Aplysia are important in regulating synaptic function.