The effects of thyroidectomy and of thyroid hormone administration on the hepatic transport of endogenous bilirubin were investigated in the Wistar R/APfd rat. Hypothyroidism resulted in an enhanced hepatic bilirubin UDP-glucuronosyltransferase activity and in a decreased p-nitrophenol transferase activity. It caused a cholestatic condition with a 50% decrease in bile flow and bile salt excretion, and an increased proportion of conjugated bilirubin in serum. The biliary output of unconjugated and monoconjugated bilirubins decreased in parallel by about 65%, whereas the excretion rate of the diconjugate dropped by only 47%, resulting in an increased di-to monoconjugate ratio in bile. Hyperthyroidism was characterized by a decreased bilirubin and an increased p-nitrophenol transferase activity, and by an augmented bilirubin output in bile. The output of unconjugated and monoconjugated bilirubins increased in parallel by about 50 or 100%, whereas the excretion of the diconjugate increased by only 20 to 50%, depending on the dose of thyroxine administered; this resulted in a decreased di-to monoconjugate ratio in bile. A linear positive relationship was found between bilirubin UDP-glucuronosyltransferase activity and the ratio of bilirubin di-to monoconjugates present in bile or formed by in vitro incubation of liver homogenates at low concentration of bilirubin (10 to 15 μM), indicating that bile pigment composition is mainly determined by the conjugation activity in the liver. The inverse relationship observed between hepatic β-glucuronidase activity and the ratio of di-to monoconjugates in bile warrants further investigation to analyze whether this enzyme activity also plays a possible role in the changes in bile pigment composition in hypo-and hyperthyroid rats.