Melatonin is a main neurohormone of the pineal gland. The effects of melatonin on the level of serum thyroid-stimulating hormone (TSH), thyroxine (T-4), triiodothyronine (T-3), nitrate, melatonin and liver superoxide dismutase (SOD) activity were examined in rats. Melatonin was injected at the dose of 10 mg/kg for 7 days, 2 h before turning the lights off. Rats were decapitated at 10:00 a.m, and 02:00 a.m., which are the times of the lowest and highest serum melatonin levels, respectively. Blood and tissue samples were collected. Decreased TSH, T-3, T-4 and nitrate levels were determined in the melatonin-injected and nighttime groups. Melatonin levels showed a diurnal rhythm. SOD activity increased in the melatonin-treated group. The results demonstrate that increased SOD activity, and reduced serum TSH, T-3, T-4 and nitrate levels correlated with the serum melatonin levels.