Biochemical, electrophysiological, and behavioral experiments suggest that the dopamine D-1 and D-2 receptor subtypes functionally interact. In rats trained to discriminate 1.0 mg/kg d-amphetamine, substitution with the D-2 agonist quinpirole (0.1-2.0 mg/kg) produces amphetamine-lever responding, whereas the D-1 agonist SKF 38393 (0.3-10.0 mg/kg) elicits only saline-appropriate responding. Combining either quinpirole (0.05-0.5 mg/kg) or SKF 38393 (0.5-10.0 mg/kg) with 0.3 mg/kg d-amphetamine results in dose-dependent increases in amphetamine-lever responding. Conversely, the D-1 antagonist SCH 23390 (0.02-0.1 mg/kg) antagonizes the discrimination produced by 0.7 mg/kg d-amphetamine. Additional combination studies examined the effect of DA receptor drugs on discrimination when quinpirole is substituted in d-amphetamine trained rats. SKF 38393 (0.5-7.0 mg/kg) fails to increase the amphetamine-appropriate lever response produced by either 0.05 or 0.2 mg/kg quinpirole. Similarly, SCH 23390 (0.01-0.1 mg/kg) fails to antagonize the amphetamine-lever responding produced by either 0.2 or 0.5 mg/kg quinpirole. Haloperidol (0.02-0.2 mg/kg) does antagonize the amphetamine-appropriate response produced by quinpirole substitution. The d-amphetamine discrimination studies indicate that stimulating D-2 receptors alone or D-1 receptors in the presence of d-amphetamine yields d-amphetamine-lever responding, and suggests that D-1/D-2 receptors can functionally interact to alter discrimination behavior. Quinpirole substitution, on the other hand, shows an insensitivity to D-1 receptor manipulations.