The administration of phenobarbitone to the rat (8 mg/100 g BW) once daily for 3 days significantly decreased the serum and tissue levels of erythromycin administered intraperitoneally (5 mg/l00 g BW). Furthermore, phenobarbitone stimulated the hepatic microsomal N-demethylation of erythromycin and increased the biliary concentration and the biliary excretion rate of the unmetabolized antibiotic. These effects were accompanied by augmented liver mass and bile flow. The possibility is discussed that erythromycin concentrates in the bile through a specialized hepatic drug transport system, activated by phenobarbitone.