To assess whether the circadian rhythm of melatonin (MT) described in umbilical cord blood of term babies is due to an active pineal in the newborn, we analyzed 119 normal neonates during the first 72 h of life. Plasma MT was measured by RIA in different neonates at different hours of the day. Statistical analysis consisted of comparison of the means of MT values grouped in two time periods of 12 h each [day period, 0900-2100 h (77 neonates); night period, 2100-0900 h (42 neonates)] and cosinor analyses to determine the existence of a circadian rhythm of MT. Mean MT levels did not vary greatly during the first 72 h of life, and no differences were found between day and night periods. These results suggest that the pineal gland in the neonate actively secretes MT, but not in a rhythmic manner, implying that the circadian rhythm of MT described previously in cord blood is a reflection of the maternal rhythm.