The present study was designed to determine whether melatonin circulates in the umbilical cord blood of healthy human newborns and whether the concentrations of this hormone follow a circadian rhythm at birth. Umbilical cord blood was collected from full-term human newborns at the time of delivery. Serum melatonin was extracted with diethylether and determined by radioimmunoassay by using radioiodinated melatonin and rabbit antimelatonin antiserum. Significant amounts of melatonin were detected in the sera obtained from umbilical cord blood. When serum melatonin concentrations of human newborns were represented over 6-h periods or hour by hour over the 24 h of 1 d, similar hormone concentrations were found consistently. These results indicate that melatonin is present in quite high levels in the umbilical cord blood, which may be, at least in part, of maternal origin. Also, our results suggest the absence of a melatonin circadian rhythm in human newborns, which may reflect an immaturity of the components involved in melatonin synthesis.