The influence of protein binding on the extraction ratio, and availability, of diazepam has been examined in the single-pass isolated perfused rat liver preparation. Binding of diazepam was varied by adjusting the concentration of albumin in the perfusate. In the absence of binding the extraction ratio of diazepam was high, 0.93–0.995. Extraction decreased dramatically as the degree of binding was increased. The data are more consistent with the “parallel-tube” model than with the “well-stirred” model, two perfusion models that have been used to describe hepatic drug elimination.