Biliary excretion has been found to be a major route of excretion of aldosterone metabolites in the rat. Four [mu]c (0.05 [mu]g) of H3-aldosterone was injected intravenously. During the first 24 hr. after injection, normal rats excreted 25% of the injected radioactivity in urine and 43% in feces. Excretion of radiometabolites was com-plete in 96 hr. with a total of 29% of the tritiated metabolites in urine and 67% in feces. Cannulation of the bile duct revealed that more than 90% of injected aldosterone is excreted in bile within 4 hr. Entero-hepatic circulation occurs since tritiated compounds appeared in urine after bile containing radiometabolites of aldosterone was given intra-gastrically. Although only 1% of the radiometabolites in bile was un-conjugated, unconjugated metabolites represented 35% of the radio-metabolites in urine and 48% in feces.