ExtractThe development and relative contribution of hepatic bilirubin conjugation with glucuronic acid, xylose, and glucose was studiedin vitroin newborn rats 1–20 days old. In adult control rats, 75% of the conjugates formed were with glucuronic acid, whereas in 1-day-old newborns, only 50% of the conjugates were with glucuronic acid (P<0.02) while xylose and glucose conjugates of bilirubin together were equal to that of glucuronic acid. Byday 4,total conjugating capacity increased to adult levels and a mature pattern of distribution. In response to phenobarbital treatment, xylose and glucose conjugation increased 4 days earlier than glucuronide conjugation and maximal induction occurred 8 days sooner for nonglucuronide conjugation than for glucuronide.SpeculationThe pattern of enzyme maturation and relative activities of the various bilirubin conjugating systems in newborn rats cannot be extrapolated to those in the human, but this study suggests that a similar study in the human or subhuman primate might demonstrate that glucose and xylose conjugation compensate, in part, for deficient glucuronide conjugation.