This report describes the effects of midazolam and beta-carboline-3-carboxylate ethyl ester (beta-CCE) on extracellular concentrations of dopamine in the nucleus accumbens of freely moving rats measured by in vivo microdialysis. The two compounds had opposite effects, midazolam (0.075 and 0.15 mg/kg i.v.) dose dependently decreasing, and beta-CCE (3 and 10 mg/kg i.p.) dose dependently increasing, dialysate concentrations of dopamine. Flumazenil (6 micrograms/kg i.v.) did not affect the efflux of dopamine but it prevented the effects of both midazolam and beta-CCE on dopamine efflux. N6-Cyclohexyladenosine (0.1, and 1 mg/kg i.p.), a selective adenosine A1 agonist, dose dependently increased the efflux of dopamine. This effect was blocked by 8-cyclopentyl-1,3-dipropylxanthine (25 mg/kg i.p.), a selective adenosine A1 receptor antagonist, a dose which given alone did not affect dopamine efflux; responses to midazolam were not affected. 3,7-Dimethyl-1-propargylxanthine (1 and 3 mg/kg i.p.), a selective adenosine A2 receptor antagonist, did not mimic the effects of beta-CCE. The results suggest that midazolam and beta-CCE modulate dopamine release in the nucleus accumbens by an action at the benzodiazepine binding site associated with the GABAA receptor complex.