Preincubation of rat forebrain membranes for 30-60 min with micromolar concentrations of the pineal hormone, melatonin, significantly inhibited forskolin-stimulated adenylate cyclase (AC) activity. Melatonin had an EC25 (concentration which inhibited AC activity by 25%) of 600 microM and caused a maximal inhibitory effect of approximately 30% at a concentration of 1000 microM. A comparison of the effects of melatonin and its analogs, 6-chloromelatonin and 2-iodomelatonin, in the striatum revealed that these halogenated drugs were 2-3 times more potent than melatonin in inhibiting AC activity. The EC25 values were 611, 226 and 189 microM for melatonin, 6-chloromelatonin and 2-iodomelatonin respectively. The receptor antagonists phentolamine (alpha-adrenergic), propranolol (beta-adrenergic), and metergoline (serotonergic) did not block the effect of melatonin in forebrain membranes. The central-type benzodiazepine (BZ) antagonist, Ro 15-1788 (flumazenil), also failed to block the inhibitory effects of melatonin, and the benzodiazepines, diazepam and Ro 5-4864, on AC activity. Evidence that inhibition of adenylate cyclase activity may be involved in the prevention of seizures suggests that the reported anticonvulsant effect of large doses of melatonin may be due to this mechanism. The greater potency of the halogenated melatonin analogs in inhibiting AC suggests that further study of their potential usefulness as anticonvulsants would be worthwhile.