Losartan, an angiotensin II receptor type 1 (AT1) antagonist, is widely used in the treatment of hypertension. Losartan alleviates oxidative stress and inhibits nitric oxide metabolism by blocking AT1 receptors. The current study investigated putative modulatory effects of Losartan on type 1 diabetes mellitus (T1DM)-induced oxidative DNA damage in corpus cavernosum (CC). Adult male Wistar rats (250 g; 15-16-week-old) were treated (i.p.) as follows (n=6/group [G]): G1-water (vehicle control), G2- Losartan (300mg L-1 drinking water), G3-single injection of Streptozotocin (i.p., 55 mg kg-1; induces pancreatic β cell destruction leading to T1DM within 72 h) and G4-G3 + Losartan. The drug was administered during weeks 4-6 after the induction of DM. The animals were anesthetized with a mixtute of ketamine and rompun (2 mL kg-1, i.m.) and sacrificed at the end of sixth week. The CC was dissected out from the penis and then either fixed in 10% formaldehyde or homogenized in lysis buffer. Paraffin sections were stained with Masson’s trichrome and structural changes were observed under a light microscope. Total antioxidant status (TAS) and total oxidant status (TOS) were quantified in a plate reader by using Trolox and H2O2 as standards, respectively. Oxidative DNA damage was evaluated by quantifying the activities of an oxidized base, 8-hydroxy-2′-deoxyguanosine (8-oxo-dG) by competitive ELISA (Narayana & Raghupathy, 2012) and immunohistochemistry (Narayana, 2010). Data are values ± S.D., and compared by one way ANOVA and LSD test. The CC in G3 (T1DM) showed degeneration of smooth muscle cells and collagen fibers compared to G1 and G2. In G4, Losartan mitigated the induced structural changes although the CC still showed degenerative changes. The TAS showed an increase in G2 (0.009 ±0.002; p<0.05), a decrease in G3 (0.002 ±0.0004; p<0.05) and a significant recovery in G4 (0.007 ±0.002; p<0.05) compared to G1 (0.006 ±0.001). Conversely, TOS was significantly increased in G3 (0.095 ±0.008; p<0.05) compared to G1 (0.029 ±0.004), G2 (0.043 ±0.014) and G4 (0.031 ±0.01). A decrease in TAS/TOS ratio in G3 (0.016 ±0.003) compared to G1 (0.20 ±0.04) indicated the induction of an oxidative stress status in the CC of T1DM rats. The ratio showed a recovery in G4 (0.26 ±0.27) compared to G3 (p<0.05). Enhanced intensity of 8-oxo-dG labeling in G3, in both nuclei and cytoplasm of the CC cells, indicated nuclear and mitochondrial oxidative DNA damage. Activity of 8-oxo-dG significantly increased in G3 (0.19 ±0.005; p<0.05) compared to G1 (0.06 ±0.001), but recovered in G4 (0.07 ±0.004; p<0.05) compared to G3. In conclusion, Losartan significantly ameliorates T1DM-induced oxidative stress, oxidative DNA damage and structural changes in the CC.