The present study aimed at demonstrating the possible changes in gonadotropins, sex hormones as well as liver and kidney functions induced by chronic administration of melatonin (Mel) 300ug/100g body weight S.C. daily for two months to intact adult male and female rats. Chronic injection of Mel. caused a significant elevation in serum luteinzing hormone (LH) and follicle stimulating hormone (FSH), but significantly reduced serum testosterone (T) level in male rats when compared with control group. In female rats administration of Mel. markedly reduced serum LH, estrogen (E) and progesterone (P) levels, but failed to change serum FSH as compared with control group. Subcutaneous injection of Mel. induced a significant elevation in serum AST level in both male and female rats. A marked reduction was also induced in serum urea level of male rats when compared with control group. The results suggest that chronic use of Mel. in rats, of either sexes, had an inhibitory effect on the reproductive system. The antireproductive effect of Mel. is due to reduction in serum T. level of male rats, and the reduction in P and E levels of female rats. This inhibition in ovarian function induced by Mel. in female rats may suggest the possible use of it as contraceptive alone or together with common contraceptives. We can also conclude that chronic administration of Mel. in rats affects liver function by elevating serum AST level, and alters kidney function, in male rats only, by reducing serum urea level.