Effects of alloxan treatment on the levels of pineal melatonin, pineal N-acetylserotonin, and serum melatonin were investigated. Male rats were housed under a photoperiod of 12 h light: 12 h darkness and a temperature of 23 ± 3°C. Three weeks after alloxan (170 mg/kg) or carrier injection (s. c.), the animals were killed at mid-light (1200 h) and mid-dark (2400 h). Pineal and serum indoles were extracted and quantified by radioimmunoassays. It was found that pineal levels of N-acetylserotonin in the diabetic rats were significantly higher (P < 0.05) than those of the controls. Conversely, pineal and serum levels of melatonin in the control rats were significantly higher (P < 0.05) than those of the alloxan-induced diabetics. Our results suggest that alloxan-induced diabetics may decrease pineal melatonin synthesis in rats by reducing the activity of hydroxyindole-O-methyltransferase, resulting in a decrease in pineal melatonin secretion.