Euthyroid, hyperthyroid, hypothyroid, and pair-fed control groups of rats were subjected to bile duct cannulation. Total biliary excretion was collected daily over a 14-day period, and analyzed for cholesterol and cholic acid. Bile volume was related to the amount of food consumed. Cholesterol excretion was increased in hyperthyroidism and decreased in hypothyroidism. Cholic acid excretion was lowered in hyperthyroidism, normal in hypothyroidism, and markedly elevated in the pair-fed series. Different means of producing the hypothyroid state showed little variation in their effect on biliary cholesterol and cholic acid excretion.