In vivo and in vitro bile pigment metabolism was studied in embryos, chicks, and mature White Leghorn hens. Biliverdin and bilirubin pigments were present in bile of embryos from the earliest stage examined (14 d). Bilirubin accounted for 2.5-11.5% of total bile pigments, with the higher percentages in the early embryo. Biliverdin was exclusively in unesterified form. In contrast, bilirubin was almost entirely (> 98%) in esterified form. Glucosides consistently predominated over glucuronides and xylosides (6:3:1). In serum, bilirubin and biliverdin were undetectable at all embryonic stages and after hatching. Bilirubin UDP-glycosyltransferase activities with UDP-glucuronic acid, UDP-glucose, and UDP-xylose were detectable in chick embryo liver and averaged 67, 72, and 102%, respectively, of the corresponding adult mean values, without significant change throughout development. We conclude that multiple bilirubin esterification systems mature early in the avian embryo. This is in marked contrast to the development of bilirubin metabolism in the mammalian fetus in which detoxication of this potentially toxic pigment is achieved by placental clearance, and bilirubin esterification matures only after delivery. These contrasting developmental patterns are consistent with an adaptive response to the different requirements in the avian embryo and mammalian fetus for self-protective detoxification of unconjugated bilirubin.