The levels of pineal and serum melatonin at mid-light and mid-dark of male rats under a photoperiod of 12h light:12 h darkness with age ranging from day 1 to day 42 postpartum were determined. At mid-dark, pineal melatonin levels were found to increase with age; when the body weight was considered, an early developmental rise (1-to-9-day old), an active period (11- to 17-day old), and a period of lower levels (after 21-day-old) were noted. Serum melatonin levels at mid-dark showed similar changes to the latter. At mid-light, this pattern of change was also present in pineal melatonin contents relative to body weight but was absent in serum melatonin levels. Our study indicated that weaning was not responsible for the pre-pubertal decline in pineal melatonin secretion. It was suggested that these changes in the secretory pattern of pineal melatonin may play a role in the development of the reproductive system in rats.