The aim of the present experiment was to investigate the possibility that alterations in dopamine D3 receptors have a role in the normalization of function that occurs following a unilateral lesion of the medial forebrain bundle induced by 6-hydroxydopamine (6-OHDA). Unilateral lesions result in an enhanced rotational response to dopamine agonists that appears to be due to an increase in stimulatory D2 receptors on the lesioned side that occurs by about 1 week postlesion. The present experiment assessed the involvement of D3 receptors in rotational behavior by testing the animals at 48 h postlesion. At this time interval, D2 receptors have not become up-regulated. In contrast, D3 receptors have been shown to be down-regulated. Rats with > or = 98% dopamine depletion induced by 6-OHDA exhibited mostly ipsilateral rotation in response to an injection of amphetamine. This rotation was not affected by pretreatment with the D3 antagonist U-99194A. Rats with 80-97% dopamine depletion exhibited mostly contralateral rotation in response to amphetamine and this rotation was blocked by pretreatment with U-99194A. In addition, a decrease in D3 receptor binding was observed by 48 h postlesion. These results support the hypothesis that the decrease in D3 receptors seen following denervation is involved in the compensatory response of the system. This may have important clinical relevance in the treatment of disorders such as Parkinson's disease and drug abuse.