Abstract INTRODUCTION: Myocardial ischemia, cerebral ischemia and myocardial infarction are the most important complications of hypertension and atherosclerotic disease in developing countries. Angiotensin converting enzyme (ACE) inhibitors are among the drugs used to treat hypertension and heart failure. Captopril is an ACE-inhibitor which also has antioxidant properties. This study was conducted to assess the antioxidant effects of Iranian Captopril on malondialdehyde (MDA), conjugated dienes (CD) and serum antioxidant capacity before and after treatment. methods: This interventional prospective single-blind study was conducted on 34 mildly hypertensive individuals and 34 patients with stage I and II heart failure. MDA, CD and serum antioxidant capacity were measured in all samples. The patients were then given 50 mg Captopril tablets 2-3 times daily. The measurements were repeated 1.5 months later. results: Comparison of mean MDA, CD and serum antioxidant capacity in hypertensive patients and patients with heart failure before and after drug administration revealed no significant difference in any of the parameters studied. Discussion: Existing evidence is suggestive of the strong antioxidative properties of Captopril in vitro, although these effects have not been borne out by some studies. In the present study, comparison of MDA, CD and serum antioxidants before and after the period of treatment with Iranian Captopril did not reveal any statistically significant difference.Keywords • Antioxidant • ACE inhibitor • High blood pressure • Heart failure • Clinical trial