1 The effects of benzodiazepine receptor antagonists on the inhibition of forskolin-stimulated adenylate cyclase (AC) activity by various benzodiazepine (BZ) and indoleamine agonists in the rat striatum were investigated. 2 A biphasic inhibition of forskolin-stimulated AC activity by the peripheral-type agonist, Ro5-4864, and a multiphasic inhibition by the non-selective BZ, diazepam, was observed. One phase of AC inhibition is consistent with a Gi-coupled receptor-mediated action, whereas the other phases appear to involve a direct effect on the enzyme itself. 3 While the central-type antagonist, flumazenil, had no effect on the ability of Ro5-4864 to inhibit AC activity, the peripheral-type receptor ligand, PK 11195, abolished the first phase of inhibition. 4 PK 11195 and pertussis toxin were found to block the inhibitory effect of various BZs and the indoleamines, melatonin and 2-iodomelatonin, on induced AC activity. 5 Saturation binding studies, conducted at 30'C with [3H]-diazepam revealed a single binding site in the rat striatum (KD = 19.3 + 0.80 nM) which significantly decreased in affinity in the presence of GTP (KD = 30.5+2.6 nM; P<0.05). No significant change in B,,,, was observed. 6 These findings indicate the presence of G,-coupled BZ receptors in the rat striatum. Thus, suppression of cyclic AMP production may contribute to the diverse neuropharmacological effects of BZs, melatonin and related drugs.