SummaryBiliary excretion of injected unconjugated and conjugated C14-bilirubin was compared in one-day-old and in adult guinea pigs. In relation to adult animals, in the newborn elimination of unconjugated pigment was reduced by more than half and that of conjugated pigment, by one-third. This indicated that in addition to impaired conjugation, the liver of newborn animals exhibits an excretory defect, but the former represents the rate-limiting step in overall pigment metabolism.