The discriminative stimulus effects of the dopamine D3 receptor-preferring agonist, S(+)-(4aR,10bR)-3,4,4a,10b-tetrahydro-4-propyl -2H, 5H-benzopyrano-[4,3-b]-1,4-oxazin-9-ol ((+)-PD 128907), were examined in rats trained to discriminate (-)-apomorphine (0.16 m/kg) from saline in a two-lever, fixed ratio 10 drug-discrimination paradigm. Both (-)-apomorphine and (+)-PD 128907 produced dose-related (-)-apomorphine-lever selection, with full substitution observed at 0.16 mg/kg, i.p. (ca. 0.5 mumol/kg). Drug-appropriate responding produced by either (-)-apomorphine or (+)-PD 128907 was antagonized by the putative dopamine D3 receptor antagonists, (1S,2R)-cis-5-methoxy-1-methyl-2-(n-propylamino)tetralin) ((+)-AJ76) and cis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino)tetralin ((+)-UH 232), as well as by the dopamine D2 receptor antagonist haloperidol. Because haloperidol was approximately 30-150-times more potent than (+)-AJ76 or (+)-UH-232 in blocking the effects of either receptor agonist, the results indicate that dopamine D2 receptors play a role in the discriminative stimulus effects of (+)-PD 128907.