Diltiazem (DTZ) is an anti-hypertensive and anti-anginal agent which is also used clinically as a metabolic inhibitor to reduce cyclosporin-A metabolism. The present communication describes an HPLC/UV method for measuring DTZ and 3 of its major metabolites. This method has a LOQ below 2.5 μg/L and within run CV's ranging from 11.3 to 1.9% at concentrations of 2.5 and 100 μg/L, and between-run CV's ranging from 10.5 to 4.9% at concentrations of 75 and 350 μg/L, respectively. The method has been applied successfully to 11 renal transplant recipients taking a variety of other drugs.