The purpose of these studies was to investigate intrahepatic changes underlying age-related decreases in bile flow by evaluating the effects of aging on bile acid-dependent and -independent flow, canalicular versus ductular flow and hepatic tight junction permeability. The isolated perfused liver was used to assess age-related changes in intrinsic hepatobiliary function without the complications of extrahepatic factors such as circulating hormones or hemodynamics. Livers from young adults (3 to 6 mo old) or senescent (22 to 26 mo old) male Sprague-Dawley rats were isolated and perfused in a nonrecirculating, hemoglobin-free system to assess oxygen uptake, bile acid-dependent and -independent bile flow, bile acid uptake, carbon 14-labeled erythritol clearance as a measure of canalicular flow, tight junction permeability and transcellular transport into bile. Rates of oxygen uptake by livers from senescent rats were significantly lower than those of young adults (75 2 8 pmol/gm/hr vs. 121 f 5 pmol/gm/hr). Age-related decreases in total bile flow were observed and were associated with similar reductions in 14C-erythritol clearance suggestive of decreased canalicular bile flow. Bile aciddependent and -independent flow was decreased by 60% and 60%, respectively, in isolated perfused livers from senescent rats. Hepatocellular uptake of taurocholate and rates of bile acid excretion also were about 50% lower in senescent than in young adult rats. Tight junction permeability and transcellular transport were assessed by monitoring appearance of tri- tiated inulin and horseradish peroxidase in bile after bolus injections of these compounds through the portal vein. Tritiated inulin appearance in bile was decreased slightly in senescent compared with young rats. No age-related differences in tight junction permeability to horseradish peroxidase were observed. In addition, biliary excretion of horseradish peroxidase in livers from senescent rats was not affected by colchicine, suggesting that microtubule-related transport was minimal or absent in the senescent rats. Taken together, these data indicate that decreased rates of bile flow in senescent compared with young rats are caused by age-related effects on intrinsic hepatobiliary ~