A neural system within the cerebral and buccal ganglia of the terrestrial mollusc Limax maximus responds to lip chemostimulation by emitting a feeding motor program (FMP) in uiuo and in uitro. We have analyzed chemically the cerebral and buccal ganglia of Limax for neurotransmitters involved in controlling expression of FMP. Dopamine was found in clusters of cells and in the neuropil of the cerebral ganglia at a concentration of 62 pmol/ganglion; a large proportion of such dopamine-containing cells projected to the lips. The buccal ganglia contained several small dopaminergic cells and large amounts of dopamine in the neuropil; the measured concentration was 10 pmol/ganglion.Exogenous dopamine applied to the cerebral and buccal ganglia in vitro between lop7 M and 3 x lop6 M excited an autoactive salivary duct motor neuron (FB) and inhibited an autoactive secretomotor neuron (BSN). Concentrations of dopamine between 3 X 10m6 M and 3 X 10e5 M triggered FMP output, with an increased probability of triggering at higher concentrations of dopamine. ADTN and SK&F38393 were potent agonists in this system, whereas ergonovine was the only potent antagonist found; none of the neuroleptics tested was effective. Thus, the Limax system shows agonist responses similar to the vertebrate Dl receptors, but its antagonist-binding properties appear to have requirements quite different from vertebrate receptors.The effects of exogenous serotonin differed from dopamine's effects; serotonin excited BSN and several buccal motor neurons, could not elicit synchronized motor program cycling, and was not efficiently blocked by ergonovine.These data suggest that dopamine is a good candidate as an endogenous triggering and sustaining transmitter for the Limax feeding motor program.