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Department of Pathophysiology, Atomic Disease Institute, Nagasaki University School of Medicine, Nagasaki 852, Japan

(Received 9 January 1978)

Melatonin has been found to inhibit the response of neonatal rat pituitary tissue to luteinizing hormone releasing hormone (LH-RH) in vitro (Martin & Klein, 1976; Martin, Engel & Klein, 1977). This inhibitory effect of melatonin has been observed previously in the pituitary gland of mature male dogs in vivo (Yamashita, Mieno, Shimizu & Yamashita, 1978), but it is not known whether the inhibition is manifested in immature animals. The response of the pituitary gland of the immature male dog to exogenous LH-RH has therefore been investigated; the rate of secretion of 17-oxosteroids by the testis in vivo was used as an index of LH release (Yamashita, 1966).

Melatonin (Sigma Chemical Co.; 100 µg/kg body weight dissolved in 0·5 ml 1·6% ethanol–isotonic saline solution) was administered into the left carotid artery of immature