P-Glycoprotein (Pgp) locates in several tissues in the living body and acts as an efflux pump for many drugs. In this study, the usefulness of intravenous rhodamine 123 (Rho123) administration as a marker for detecting the inducing effect of Pgp by drugs was identified, and the relationship between excretion clearances of Rho123 via Pgp and its expression during treatment with the representative Pgp inducers rifampicin (RFP), dexamethasone (DEX) and St. John's Wort (SJW) were examined in rat liver, intestine and kidney. After pretreatment with RFP (10 mg/kg/d) for 4 d, DEX (50 mg/kg/d) for 4 d or SJW (15 mg/kg/d) for 7 d orally, the biliary excretion of Rho123 after intravenous administration (0.2 mg/kg) increased significantly by 40%, 55% and 14%, respectively, and the intestinal excretion increased significantly by 24%, 50% and 27%, respectively, as compared with the controls. In contrast, there were no notable changes in the urinary excretion of Rho123 among rats that received these inducers. Western blot analysis with a monoclonal antibody for Pgp (C219) showed that Pgp levels in the small intestine and liver in the inducer-treated rats increased markedly as compared with the controls. In addition, there was a significant correlation between the induction levels of Pgp in the liver or small intestine and their clearance ratios (r2=0.7583, p<0.05), but not in the kidney. These observations suggest that the excretion clearances of Rho123 from blood circulation to the small intestine or to the bile after its intravenous administration are useful indicators to assess the Pgp function in the presence of Pgp inducers.