Abstract: Male rat pups were given a single dose of melatonin on day 5 of age. On days 30, 45, and 60, prolactin (PRL), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and testosterone (T) were measured in serum and monoamine oxidase A (MAO A) activity, monoamine oxidase B (MAO B), and phenylethanolamine-N-methyl transferase (PNMT) activity were measured in the hypothalamus. Melatonin administration increased serum PRL levels at all ages studied. Serum LH levels were decreased in the melatonin treated group on day 30, but levels were elevated on days 45 and 60 of age as compared to controls. LH response to luteinizing hormone-releasing hormone (LHRH) only increased in melatonin treated animals at 30 days of age. Serum T levels decreased with melatonin treatment at 30 days of age, but increased on days 45 and 60 of age. T response to human chorionic gonadotropin (hCG) was blunted by melatonin treatment at 30 days of life. Hypothalamic MAO A activity increased, after neonatal melatonin administration, at 30 and 45 days of age and decreased at 60 days of life. Early neonatal melatonin administration increased MAO B and PNMT activities on day 45. These findings suggest that neonatal melatonin administration induces an earlier sexual maturation in male rats, possibly related to PRL, LH, MAO, and PNMT increases.