To clarify the role of dopamine D1 and D2 receptors in the volume-induced micturition reflex, conscious, female rats were investigated cystometrically before and after intravenous administration of SKF 38393 (a selective D1 receptor agonist), SCH 23390 (a selective D1 receptor antagonist), quinpirole (a selective D2 receptor agonist), and remoxipride (a selective D2 receptor antagonist). The effect of quinpirole was also investigated in the presence of remoxipride. Intravenous administration of SKF 38393 (0.01-3.0 mg/kg) did not affect any cystometric parameters investigated. On the other hand, SCH 23390 (0.1-1.0 mg/kg i.v.) reduced bladder capacity and micturition volumes and increased the micturition pressure in a dose-dependent manner. Quinpirole (0.01-0.1 mg/kg) given intravenously, dose-dependently decreased bladder capacity and micturition volumes. Pre-treatment with remoxipride (1.0 mg/kg i.v.) significantly attenuated the effect of quinpirole (0.1 mg/kg i.v.). Remoxipride (0.1-1.0 mg i.v.) itself did not cause any significant changes in the cystometric parameters. These results suggest that in conscious rats, D1 receptors tonically inhibit the micturition reflex and that D2 receptors are involved in facilitation of the micturition reflex. It may be speculated that detrusor hyperreflexia associated with Parkinson's disease results from activation failure of D1 receptors and that administration of D2 receptor agonists might worsen the condition.