RESUMO
This study was carried out to evaluate the hormonal and some biochemical changes which might be caused by administration of melatonin [Mt] daily for five months to 36 albino rats of both sexes. Long-term administration of doses equivalent to human dose range of melatonin caused an increase in testosterone, a decrease in estradiol [E2] levels in males with no change in the levels of both hormones in females, an increase in follicular stimulating hormones [FSH] and luteinizing hormone [LH] and a decrease in prolactin [PRL] levels in both sexes. Serum glutamicoxalo-acetic transaminase [GOT], glutamic- pyruvic transaminase [GPT] and sorbitol dehydrogenase [SDH] showed a significant increase from the third month onwards, while serum urea and creatinine showed a significant increase only at the fifth month