This study was performed to ascertain whether testosterone metabolism is altered in male rats with portal bypass, and whether such changes could contribute to the reduction in serum testosterone concentration and raised serum estrogen levels that are observed in this situation. The metabolic clearance rate of testosterone was determined by a prime-dose constant-infusion technique in male rats subjected to complete portal vein ligation and in sham-operated controls. Testosterone clearance was similar in rats with portal vein ligation and control rats (9.01 +/- 2.29 and 8.26 +/- 2.83 ml/min, respectively) but the clearance per gram of liver was greater in rats with portal vein ligation than in controls (1.18 +/- 0.18 versus 0.68 +/- 0.24 ml/min.g liver, p less than 0.0001). After 180 min of [3H]testosterone infusion, [3H]estradiol comprised 1.2% of plasma total radioactivity in male controls but was increased to 11% in rats with portal vein ligation (p less than 0.005). Similarly, biliary excretion of [3H]estradiol was eightfold greater in male rats with portal vein ligation compared with controls (p less than 0.001). In control male rats, the major metabolites of testosterone present in bile were 2 alpha-hydroxytestosterone, 16 alpha-hydroxytestosterone, and 7 alpha-hydroxytestosterone. Portal bypass was associated with reduced biliary excretion of 2 alpha-hydroxytestosterone and 16 alpha-hydroxytestosterone to approximately 50% of control, but there was no change in the excretion of 7 alpha-hydroxytestosterone. Conversely, portal bypass was associated with increased formation of dihydrotestosterone, indicating stimulated activity of testosterone 5 alpha-reductase. It is concluded that portal bypass in male rats is associated with altered pathways of testosterone metabolism and, in particular, with increased aromatization of testosterone to estradiol. The site of such estradiol formation has not been determined by this in vivo study. However, selective changes occurred in the regiospecific and stereospecific hydroxylation pathways of testosterone and in 5 alpha-reductase activity after portal bypass in male rats. It is concluded that portal bypass, in the absence of parenchymal liver damage, results in demasculinization and feminization of C19 steroid metabolism in the male rat liver. These metabolic changes could be revelant to the pathogenesis of changes in sexual characteristics in cirrhosis.