The kinetics of bromosulfophthalein clearance from plasma, secretion into bile, and, by difference, storage in hepatic cells, were determined, using normal livers and blood from normal rats in the isolated perfused rat liver system. Two suspected uremic 'toxins', urea and guanidinosuccinic acid, were then administered in order to determine if either of these substances would independently alter the kinetics under study. Results indicated that these two compounds have little or no effect on liver function on an acute basis. However, because of the excellent reproducibility obtained, it is believed that this method can be of significant use in the further screening of substances from uremic patients which may be interfering seriously with liver functions.