Treatments with drugs to up-regulate specific receptors is a strategy often employed in mechanism of action studies. In this type of experiment, changes in the numbers of receptors and concomitant changes in an animal's sensitivity to the drug have been used as evidence for the participation of the binding site in the behavior. In these studies, to test for the role of D1 and D2 receptors in apomorphine-induced stereotypy (AIS), dopamine binding sites were up-regulated by appropriate pre-treatments and the ability of these pre-treatments to alter AIS was subsequently investigated. In the first experiment, 19 days of pre-treatment with SCH 23390 or haloperidol selectively increased by 35 and 40% the numbers of striatal D1 and D2 binding sites, respectively, without affecting their affinities. However, when challenged with apomorphine, only the animals pre-treated with the D2 antagonist showed behavioral supersensitivity. In the second experiment, reserpine pre-treatment (30 mg/kg IP, 24-hr pre-test) increased the numbers of D1 binding sites by 18%, but did not significantly alter the numbers of striatal D2 binding sites. Behaviorally, these rats were supersensitive to apomorphine's stereotypy-inducing effects; however, they also showed an increased sensitivity to the ability of either haloperidol or SCH 23390 to block AIS. Moreover, this blockade was only attenuated by a D2 (but not a D1) agonist. Collectively, these data suggest that AIS is mediated by both D1 and D2 binding sites, but that D2 binding sites have a more important role.