The effects of melatonin (3 & 30 mg/kg, p.o. daily for 3 months) on serum glucose, insulin, cortisol, testesterone, T3 and T4 levels in adult male rats were investigated. Rats were divided into 3 groups, first and second groups received 3 & 30 mg/ kg melatonin orally as a morining daily dose for 3 months respectively. The third group received solvent only and served as control. Melatonin at 3 mg/ kg dose level enhanced carbohydrate metabolism as it increased serum glucose and insulin levels. It also decreased testosterone level. Both dose levels had no significant effect on serum T3 and T4 levels. However, both dose levels increased serum cortisol level. Melatonin at a dose of 30 mg/ kg level had no significant effect on serum glucose, insulin, and testosterone levels. It is concluded that melatonin at 3 mg/ kg dose level enhances carbohydrate metabolism and has antigonadotrophic action. The data suggest that both dose levels are equal in their action on thyroid function and cortisol level. However the lower dose is more effective on carbohydrate metabolism and testosterone than the higher dose.