Feeding behavior in Aplysia can be modified by operant conditioning in which contingent reinforcement is conveyed by the esophageal nerve (E n.). A neuronal analogue of this conditioning in the isolated buccal ganglia was developed by using stimulation of E n. as an analogue of contingent reinforcement. Previous studies indicated that E n. may release dopamine. We used a dopamine antagonist (methylergonovine) to investigate whether dopamine mediated the enhancement of motor patterns in the analogue of operant conditioning. Methylergonovine blocked synaptic connections from the reinforcement pathway and the contingent-dependent enhancement of the reinforced pattern. These results suggest that dopamine mediates at least part of the neuronal modifications induced by contingent reinforcement.