The effect of kepone on the biliary excretion of [ 14C]imipramine (IMP) was studied using isolated perfused rat liver preparations obtained from control and kepone-pretreated animals. Biliary excretion of endogenously formed polar metabolites of IMP was markedly suppressed (70%) by preexposure to kepone at 200 ppm in daily ration for 8 days. In vitro addition of kepone at 5 × 10 −4 m concentration to the perfusate of untreated liver preparations elicited a 20% decrease in the biliary excretion of endogenously formed metabolites. At concentrations up to 5 × 10 −5 m, kepone was without any discernible effect. The marked suppression of biliary elimination of endogenously formed metabolites is unrelated to the rate of metabolism of IMP. Furthermore, the biliary excretion of exogenously provided polar metabolites of IMP was also suppressed (61%) by preexposure to kepone. This suppression of biliary excretion of IMP metabolites was accompanied by an increase in the bile flow suggesting a dichotomy between bile secretion and biliary excretion. In vitro addition of kepone also resulted in a suppresion (48%) of biliary elimination of exogenously provided metabolites of IMP. In the experiments in which kepone was added, bile flow was unaffected. Biliary excretion of sulfobromophthalein (BSP) was unaffected by preexposure to kepone, suggesting that kepone-induced impairment of hepatobiliary function may not be detected by BSP clearance tests. In addition, different mechanisms might be involved in the transport of IMP metabolites and BSP across the biliary canaliculi. From these results, it is suggested that the kepone-induced impairment of hepatobiliary function may be located at the site of transfer of metabolites across the bile canaliculi.