An isolated rat liver perfusion system has been utilized in a study of the biliary excretion of estrone glucuronide. The estrogen was initially shown to be excreted without prior metabolism. Disappearance from the medium was rapid and biliary concentrations exceeded that in the medium by more than a thousand-fold. Disappearance rates were decreased when medium estrone glucuronide concentrations exceeded 0.29 mM. Inhibition by other steroidal conjugates, testosterone glucuronide, 2-methoxyestrone (3-hydroxy-2-methoxy-estra-1,3,5(10)-trien-17-one glucuronide and 2-hydroxyestrone (2,3-dihydroxyestra-1,3,5(10)-trien-17-one) glutathione, was also demonstrated. Phenolphthalein glucuronide, at 10 times the molar concentration of estrone glucuronide, did not affect the medium clearance of the latter compound. These findings indicate the possibility of utilizing this system for further studies of possible interactions by other organic compounds for excretion via the biliary route.