A subpopulation of projection neurons in the nucleus accumbens uses the neuropeptide enkephalin as a neurotransmitter. The synthesis of enkephalin in striatal neurons is regulated by dopaminergic inputs. In the present study quantitative in situ hybridization histochemistry was used to examine the effects of unilateral 6-hydroxydopamine lesions of the ascending dopaminergic fibres on levels of enkephalin mRNA in subregions of the nucleus accumbens. It was found that the levels of enkephalin mRNA were increased throughout the nucleus after the lesion. However, the increase appeared to be much higher in rostral parts of the nucleus than in caudal parts, indicating regional differences in the effects of blockade of the dopaminergic neurotransmission. Possible causative factors for these differences are discussed.