The partial reinforcement extinction effect (PREE) refers to the increased resistance to extinction observed in animals trained on a partial reinforcement (PR) schedule compared with those trained on a schedule of continuous reinforcement (CR). It has been suggested that the PREE is dependent upon the integrity of the septo-hippocampal system, but recent evidence has indicated that the role originally proposed for the lateral septal nucleus may in fact be subserved by the nucleus accumbens. Experiment I therefore tested the effects of electrolytic lesions of the nucleus accumbens on the PREE. These lesions abolished the PREE, the abolition resulting from a decreased rate of extinction in the lesion CR rats coupled with an increased rate of extinction in the PR rats. These results clearly implicate the nucleus accumbens in the development of the PREE, and suggest that theoretical models of the PREE based simply upon consideration of septohippocampal interactions need radical revision. The lesion also enhanced running speeds in acquisition in both the CR and the PR groups. Experiment II therefore assessed spontaneous locomotor activity and the locomotor response to amphetamine challenge at two doses. The lesion produced no increase in spontaneous locomotion; an enhanced increase in response to 1 mg/kg amphetamine; and no changes in the stereotyped behaviours induced by 10 mg/kg amphetamine.