Transport by ATP-dependent efflux pumps such as P-glycoprotein is an increasingly recognized determinant of drug disposition. P-glycoprotein does not only contribute to multidrug resistance (MDR) in tumor cells, it is also expressed in normal tissues with excretory function such as liver, kidney and intestine. Apical expression of P-glycoprotein in such tissues results in reduced drug absorption from the gastrointestinal tract and enhanced drug elimination into bile and urine. Moreover, expression of P-glycoprotein in the endothelial cells of the blood-brain barrier prevents entry of certain drugs into the central nervous system. Human P-glycoprotein has been shown to transport a wide range of structurally unrelated drugs such as digoxin, quinidine, cyclosporine and HIV-1 protease inhibitors. Drug administration to P-glycoprotein knock-out and control mice provided data on the importance of P-glycoprotein for absorption after oral administration and penetration through the blood-brain barrier. Moreover, P-glycoprotein knock-out mice were used to identify inhibition of P-glycoprotein-mediated transport as a mechanism for drug interactions such as the digoxin-quinidine interaction. Studies in humans indicate a particular importance of intestinal P-glycoprotein for bioavailability of the immunosuppressant cyclosporine. Moreover, induction of intestinal P-glycoprotein by rifampin has now been identified as the major underlying mechanism of reduced digoxin plasma concentrations during concomitant rifampin therapy. In summary, P-glycoprotein functions as a defense mechanism, which determines bioavailability and CNS concentrations of drugs. Modification of P-glycoprotein function is an important underlying mechanism of drug interactions in humans. However, disposition of a drug and its metabolites frequently is not only determined by P-glycoprotein, but also by drug-metabolizing enzymes and possibly by drug transporters other than P-glycoprotein [e.g. members of the MRP family (MRP = multidrug resistance-associated proteins)].