The serum concentration of melatonin, serotonin, and N-acetylserotonin were measured by RIA procedures in 28 infants aged 1 week to 9 months. Blood specimens were obtained at 12:00 hr and 24:00 hr. A day-night difference in serum serotonin was present immediately after birth. A significant (P less than 0.001) decrease in serum serotonin concentrations at 12:00 hr and 24:00 hr was observed from the first month of age to the third to ninth month of age. A significant (P less than 0.05) difference in day-night N-acetylserotonin concentration is first seen at age 1-3 months. Serum melatonin concentrations, though detectable, did not show any day-night difference at birth. Melatonin concentrations progressively increased up to the third month of age, and a significant (P less than 0.01) day-night difference appeared thereafter. The results indicate that in humans the circadian organization for serotonin already exists at birth, and the circadian melatonin rhythm develops after birth.