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The rate of secretion of 17-oxosteroids by the testes of anaesthetized dogs in vivo was used as an index of LH secretion. Intracarotid injection of luteinizing hormone releasing hormone (LH-RH, 1, 5 or 10 µg/kg body wt) resulted in an increase in the testicular 17-oxosteroid secretion which was roughly proportional to the dose administered and which reached a maximum 60 min after the injection. Testicular output of 17-oxosteroids was unaffected by administration of melatonin (10 or 100 µg/kg body wt) into the carotid artery. When LH-RH (5 µg/kg) was injected into the carotid artery 3 h after intracarotid injection of melatonin (10 or 100 µg/kg), the testicular response to LH-RH was considerably diminished. Pretreatment with melatonin (100 µg/kg) did not alter the testicular response to human chorionic gonadotrophin (20 i.u./kg body wt) given i.v.

It is concluded that melatonin may act directly on the anterior pituitary gland in dogs to inhibit the LH-RH-induced release of LH.