]The conjugation of bilirubin with glucuronic acid in homogenates of tissue from fetal rats was studied. Despite the presence of excess amounts of bilirubin and uridine diphosphoglucuronic acid, conjugation activity was not detectable in liver and kidney from animals younger than 5 days prepartum. Synthesis of bilirubin glucuronide was first observed at that time, rapidly reached 50% of normal levels by birth, and achieved normal levels 5 to 6 weeks postpartum. No evidence for inhibitors or the absence of cofactors could be demonstrated in the fetal liver. Uridine diphosphoglucuronic acid levels were also low in that tissue. Regardless of the age of the animal, conjugation activity was not observed in homogenates of stomach, intestine or placental tissue. The evidence presented suggests that glucuronyl trans-ferase activity and the production of diphosphoglucuronic acid is impaired in liver and kidney from the newborn and the premature animal.