E levated serum estradiol concentrations and specific changes in the biliary excretion of some androstenedione metabolites have been reported in male rats with portal bypass produced by portal vein ligation (PVL). In this study, the hypothesis that male-specific forms of cytochrome P-450 are altered after PVL was tested by measuring microsomal steroid hydroxylase activities. Consistent with earlier findings in the intact animal, androstenedione 16 alpha-hydroxylase activity was reduced after PVL to 44% of control (P less than 0.05). Other pathways of androstenedione hydroxylation, and total estrogen formation (after androstenedione aromatization) were unchanged. Although total estrogen formation was not different, a sevenfold greater proportion of estradiol was produced in PVL rat microsomes. Additional experiments revealed that PVL selectively reduced the rate of microsomal estradiol 16 alpha-hydroxylation (to 56% of control, P less than 0.02). Levels of cytochrome P-450UT-A, the microsomal steroid 16 alpha-hydroxylase, were lower after PVL (56% of control, P less than 0.05), so that the present observations are consistent with the earlier suggestion that portal bypass is associated with the selective downregulation of this enzyme. Since downregulation of cytochrome P-450UT-A also occurs in experimental hepatic cirrhosis, portal hypertension may well contribute significantly to altered drug metabolism in liver disease. Impaired hepatic elimination of androstenedione by hydroxylation may indirectly enhance extrahepatic aromatization of the androgen. The decreased activity of hepatic estradiol 16 alpha-hydroxylation after PVL would enhance the accumulation of estradiol, the biologically more potent estrogen.