1. The metabolism of trenbolone acetate, 17 beta-acetoxyestra-4,9,11-trien-3-one (TBA), an anabolic compound used as a growth promoter, was compared in rat and cow. 2. [6,7-3H] TBA was injected i.v. into rats and a heifer, and bile was collected for 24 h. In both species, the bile was the major route of excretion. TBA undergoes an extensive hydrolysis to 17 beta-hydroxyestra-4,9,11-trien-3-one and the unchanged compound was not detected, but subsequent major metabolic pathways are different in the two species. 3. In the rat, oxidation of the 17 beta-hydroxyl to the 17-oxo group and hydroxylation in the 16 alpha-position are the major routes. The three major metabolites are 17 beta-hydroxyestra-4,9,11-trien-3-one, 16 alpha, 17 beta-dihydroxyestra-4,9,11-trien-3-one and 16 alpha-hydroxyestra-4,9,11-trien-3, 17-dione. 4. In the heifer, 17 alpha-epimerization is the major pathway and the main metabolite is the 17 alpha-hydroxyestra-4,9,11-trien-3-one. 5. In both species, estra-4,9,11-trien-3,17-dione and the other metabolites, resulting either from hydroxylation in 1, 2, 6 beta, 16 alpha or 16 beta positions, or from aromatization of the A ring, were minor products. 6. Overall, 60% of the 3-oxotriene structures identified in the rat bile were 17 beta-hydroxylated and the remainder were 17-keto metabolites, whereas in the heifer bile 90% were 17 alpha-hydroxylated compounds. 7. Thus, in bovine species, the major pathway is similar to those of testosterone or 17 beta-estradiol which are mainly excreted as their 17 alpha-epimers. The epimerization strongly decreases the biological potency, as with the natural 17 beta-hormones, and leads to detoxication of tissue residues.