Quinapril-d5 is intended for use as an internal standard for the quantification of quinapril by GC- or LC-MS. Quinapril is a prodrug form of the angiotensin converting enzyme (ACE) inhibitor quinaprilat. In vivo, quinapril reduces mean arterial pressure in renal hypertensive and spontaneously hypertensive rats. It inhibits angiotensin I-induced pressor responses in normotensive rats and dogs. Quinapril prevents left ventricular heart failure in CHF 14.6 cardiomyopathic hamsters. Formulations containing quinapril have been used in the treatment of hypertension, heart failure, and diabetic nephropathy.