Small doses of melatonin (0.1-100 ng), injected into the nucleus accumbens of rats, decreased locomotor activity and rearing, and increased grooming and sniffing behaviour when the animals were tested in small test-cages. Larger doses of melatonin appeared to be less effective. The action of melatonin is apparently not mediated by dopaminergic systems, because the behavioural changes were not antagonized by local pretreatment with haloperidol or sulpiride. Injection of serotonin antagonists (methysergide and cyproheptadine) into the nucleus accumbens resulted in similar behavioural changes as was found after treatment with melatonin. Treatment with serotonin and various antidepressant drugs (zimelidine, mianserin, nortriptyline, clomipramine, desipramine) injected into the nucleus accumbens, completely inhibited the melatonin-induced behavioural responses. The antidepressants did not significantly interfere with the decrease of locomotor activity and rearing induced by injection of small doses of the dopamine agonist, apomorphine, into the nucleus accumbens. These results suggest that there is an interrelationship between melatonin and serotonin systems in the nucleus accumbens and showed that various antidepressant drugs, similar to serotonin, antagonized the behavioural effects of melatonin after injection into the nucleus accumbens.