Multiple indicator dilution studies of warfarin uptake were carried out on isolated perfused rat livers in the presence and absence of bovine serum albumin (BSA) in the perfusate using normal rats and Nagase analbuminemic rats (NAR). A distributed model was fitted to the dilution data and estimates of influx, efflux, and sequestration rate constants were obtained. In both groups of rats, the intrinsic clearance for unidirectional hepatic uptake (CLint,influx) of warfarin in the presence of 1.6 g/dl BSA was approximately 37-45% of that in the absence of BSA, while the unbound fraction of warfarin with 1.6 g/dl BSA in the perfusate was only 4.2% of that in the absence of BSA. Thus the degree of BSA-induced reduction of the value of CLint,influx and that of the unbound fraction are different. From these observations, it was found that the hepatic uptake of warfarin is not driven solely by the unbound concentration of warfarin, supporting the recent concepts of albumin-mediated transport for tightly albumin bound ligands as reported by Ockner et al. In addition, the fact that the same hepatic uptake mechanism of warfarin was also observed in NAR suggested that the hepatic uptake of warfarin may not necessarily require a special albumin receptor on the hepatocyte surface.