The metabolism of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one has been studied after intravenous administration to bile duct-cannulated rats. Very rapid and substantial conversion of the 15-ketosterol to polar biliary metabolites was observed in both male and female rats. For example, upon intravenous injection of [4-14C]5 alpha-cholest-8(14)-en-3 beta-ol-15-one to male bile duct-cannulated rats, approximately 86% of the administered 14C was recovered in bile in the first 38 h. Of the total amount of 14C recovered in bile in 38 h, approximately 50% was excreted in bile in the first 70 min and approximately 90% was excreted within 8 h after the injection of the 15-ketosterol. A substantial fraction of the polar biliary metabolites was shown to undergo enterohepatic circulation. Of the radioactivity derived from the labeled 15-ketosterol which was not recovered in bile or other excreta at 48 h after the intravenous administration of the 15-ketosterol, most (approximately 79%) was recovered in the form of cholesterol and cholesteryl esters of blood and the various tissues. The very substantial and rapid biliary excretion of polar metabolites of the 15-ketosterol (or of cholesterol derived from the 15-ketosterol), coupled with inhibition of the intestinal absorption of cholesterol by the 15-ketosterol, may contribute to the overall hypocholesterolemic action of the 15-ketosterol which has been observed in rodents and in nonhuman primates by providing a metabolic pathway(s) wherein a substantial fraction of the absorbed 15-ketosterol is rapidly removed from the body by biliary excretion in the form of polar metabolites.