Hepatic artery and portal vein blood flows were assessed separately in anesthetized rats using radiolabeled taurocholate. The ratio of portal vein blood flow to total hepatic blood flow was calculated from measurements of taurocholate concentrations of portal vein, hepatic artery and hepatic vein, and of taurocholate hepatic extraction ratio. Animals were administered [14C]taurocholate intraperitoneally as a label of the taurocholate pool 24 hr before the experiments. In steady-state conditions, serum [14C]taurocholate activities were used as an index of taurocholate relative concentration and the hepatic extraction ratio of taurocholate was estimated using [3H]taurocholate infused into the systemic blood stream. Total hepatic blood flow was estimated by the Fick's principle using indocyanine green. The mean hepatic blood flow was 13.9 ml X min-1 (S.E. 1.9), and the mean ratio of portal blood flow to hepatic blood flow was 72.2% (S.E. 6.7). It is concluded that taurocholate may be used to assess separately portal venous and hepatic arterial blood flow in the rat.