Organic nitrates are nitric oxide donors used as vasodilators in cardiovascular diseases. Hemoglobin binds to NO with great affinity and forms methemoglobin by oxidation in the erythrocyte, as a consequence an increase in reactive oxygen species (ROS) as superoxide and reactive nitrogen species (RNS) as peroxynitrite which may increase oxidative stress and lipid peroxidation and alter the erythrocyte antioxidant defense system. The aim of the present work is to study some biochemical effects of organic nitrates on hemoglobin and to evaluate the role of vitamin C (Ascorbic acid) as an antioxidant agent against the biochemical changes induced by different doses of organic nitrates (isosorbide mononitrate) given orally to male rats. The results revealed that treatment with organic nitrates caused marked elevation in the level of lipid peroxidation (estimated by the amount of MDA) and a decline in SOD, GSH and CAT activities accompanied with an increase in the rate of hemoglobin autoxidation and a decrease in iron and TIBC. The results also demonstrated a marked increase in hemoglobin derivatives as methemoglobin and carboxyhemoglobin. Treatment with vitamin C significantly reduced the changes caused by organic nitrates in all examined parameters. These results indicate that alterations caused by organic nitrates are connected with free radicals generation and used antioxidant effective