We previously found that the uptake of warfarin in the presence of albumin by perfused rat liver could not be explained simply by the unbound warfarin concentration. The aim of the present study is to develop a kinetic model to account for this albumin-mediated uptake of warfarin. Single circulation indicator dilution studies on warfarin uptake were carried out in the isolated perfused rat liver in the absence and presence of various concentrations of bovine serum albumin (BSA) in the perfusate. A distributed model was fitted to the dilution data and the estimates of the influx, efflux, and sequestration rate constants were obtained. The results showed that the predicted concentration of the unbound warfarin is not high enough to explain the observed uptake rate; the liver cell surface appears to reduce the binding affinity of warfarin for BSA to 1/20 of that observed in vitro. A kinetic model which considers the interaction between albumin and the liver cell surface was fitted to the uptake rates of warfarin over a wide range of BSA concentration. The model gave a dissociation constant of the cell surface for albumin of 160 microM, which is comparable with those reported by others for the hepatic extractions of free fatty acids and rose bengal. Based on this kinetic model, the contributions of the unbound and bound warfarin to its hepatic uptake were estimated, and the bound warfarin was found to contribute most in the physiological albumin concentration range.