The pineal hormone melatonin influences the neurohypophysial hormone release from the isolated hypothalamus in vitro through the effect on the cholinergic pathways as well as the biosynthesis of prostaglandins. The aim of the present study was, therefore, to investigate the effects of melatonin (0.5, 1, or 5 ng) administered in vivo on the vasopressin and oxytocin release as well as to examine whether similar interactions between melatonin and acetylcholine or prostaglandins occur in vivo. In the initial study on the effect of melatonin male Sprague-Dawley rats were implanted under anaesthesia with an arterial and venous cannula. Melatonin in a dose of 0.5 ng injected intravenously had no effect on plasma vasopressin concentration. The higher dose of 1 ng caused a significant decrease in vasopressin release 10 min after injection, whereas 5 ng melatonin caused an increase in plasma hormone concentrations, the difference being significant 20 min after injection. No significant effects of melatonin on the oxytocin release was found. In the second study in which an I.C.V. cannula was additionally implanted, the cholinergic muscarinic receptor antagonist atropine (10 microg) injected I.C.V. abolished the melatonin-induced effects on plasma vasopressin level. On the other hand, a cyclo-oxygenase inhibitor ibuprofen (75 microg) injected I.C.V. blocked the vasopressin release induced by 5 ng melatonin and reversed the inhibitory effect of 1 ng melatonin. These results demonstrate that melatonin affects the neurosecretory function of the hypothalamo-neurohypophysial complex in vivo possibly via mechanisms involving cholinergic transmission and/or prostaglandin biosynthesis.