The present invention provides a method for improving the bioavailability of a renin inhibitor, preferably, of a delta-amino-gamma-hydroxy-omega-aryl-alkanoic acid derivative, which method comprises co-administering to a mammal, especially a human, in need of such treatment, a combination of a renin inhibitor, or a pharmaceutically acceptable salt thereof, and an MDR1 inhibitor selected from a non-pharmacologically active compound.