Melatonin (MEL) is the main neuro-hormone of the pineal gland. The main physiological function of MEL is to modulate variations of the light-dark cycle and seasonal periodicity on various physiological functions. Zinc (Zn) is required as a catalytic component for more than 200 enzymes and is a structural constituent of many proteins, hormones, neuropeptides, and receptors. In addition, tissue Zn levels have been found to be altered after pinealectomy in rats. Recent findings have shown that MEL can modulate Zn turnover. In the present study, we investigated the effect of MEL treatment on the tissue and serum Zn levels. Male Spraque-Dawley rats (n = 24) were used. Rats received seven daily subcutaneous injections of MEL (10 mg/kg). After 7 days, rats were decapitated at 10:00 a.m. and 02:00 a.m., which were the periods of lowest and highest scrum MEL levels, respectively. Blood and tissue Samples were collected, and Zn levels were measured by atomic absorption spectrophotometry. In conclusion, MEL increased Zn levels in the salivary glands and small intestine. Daytime liver Zn levels and nighttime serum Zn levels increased in MEL-treated group. There was no significant difference in brain after MEL treatment. Nighttime serum MEL levels were higher than daytime in both control and MEL-treated group. In addition, MEL levels in MEL-treated groups were significantly higher than those of control groups. It is clear that there is a tight relationship between Zn and MEL. J. Trace Elem. Exp. Med. 15:1-8, 2002. (C) 2002 Wiley-Liss, Inc.