Unconjugated, mono- and diconjugated bilirubin was measured in rat plasma and intestine during perinatal development. Bilirubin monoconjugates were present in the intestines of 18-day-old fetuses, while diconjugates appeared on the 20th day. The concentration of conjugates increased rapidly, reaching 312 mumol/g of intestinal tissue 1 day after birth. The absolute concentration of unconjugated bilirubin increased in parallel. On the 18th day of fetal life, about 80% of the pigment in the intestine was unconjugated, while conjugates largely predominated from the 20th day onwards. Conjugation and/or excretion of bilirubin in the intestine seems therefore to develop rapidly during the last days of life. The plasma concentration of total bilirubin in 18-day-old fetuses was about 2-3 times higher than in adult serum, and it increased slightly during the following days. 2-7% of the pigment was conjugated. Transplacental clearance of bilirubin seems therefore to be deficient in this species. On the 1st day of extrauterine life, bilirubin concentration increased sharply in plasma, reaching 14.76 mumol/l, 38% in the conjugated form. This finding suggests that, unlike humans, an excretory defect is predominant in the neonatal rat.