Objective: To evaluate the effects of captopril (CAP),an angiotensin converting enzyme inhibitor (ACEI), on oxidative stress,liver enzymes, cardiovascular damage, and micronucleus (MN) frequency in streptozotocin-induced diabetes mellitus (STZ-DM). Material and Methods: A total of 24 young Sprague-Dawley male rats weredivided into three groups as control, STZ, and STZ+CAP. The rats withDM, induced by way of a single dose injection of STZ (45 mg/kg), received CAP 50 mg/L/day for eight weeks. At the end of this period,liver and cardiovascular damage was evaluated from the level of creatine kinase (CK), creatine kinase isoenzyme-MB (CK-MB), alanineaminotransferase (ALT), aspartate aminotransferase (AST), and lactatedehydrogenase (LDH) and the level of lipid peroxidation was evaluated using the thiobarbituric acid reactive substances (TBARS), glutathione (GSH), and total thiol content (T-SH). The MN frequency wasalso examined. Results: STZ was found to cause significant increasesin the TBARS and LDH levels, ALT and AST activity, and MN frequency and a decrease in the GSH levels, while CAP increased T-SHproduction and decreased LDH activity and MN frequency. However,CAP increased the activities of ALT and AST. Conclusion: Our studyresults suggest that CAP produces antioxidant effects in rats with STZinduced DM and long-term use of CAP can correct the complicationsof diabetes at the cellular level, such as cell and tissue damage, as wellas causing liver damage. Based on these results, diabetes seems to affect the liver, kidneys, and heart to varying degrees, although the liveris the most extensively affected organ.