This study aimed to investigate the effect of aminoguanidine (AG) against testicular damage streptozotocin (STZ) induced diabetes. Thirty two rats were separated into four groups: control, AG, STZ and STZ+AG. In the STZ group, 12.5±1.3% of tubules were seen as containing sloughed spermatogenic cells into the lumen, 28.7±1.8% of tubules were atrophic, 46.2±2.1% of tubules were degenerative and 8.5±0.9% of tubules contained giant cells. Statistically, the affected tubule number was significantly lower in the STZ+AG group than in the STZ group. Intensely stained caspase-3 cells showed a statistically significant increase in the STZ group, while it decreased in the STZ+AG group. The enzyme activities of catalase (CAT), superoxide dismutase (SOD) and glutathione (GSH) level decreased and the level of malondialdehyde (MDA) and nitric oxide (NO) increased in the STZ group, while AG treated diabetic rats showed an increase of CAT, SOD activity and GSH level and a decrease in MDA and NO levels. This study shows that the oxidative stress, increased NO level and apoptotic cell death play an important role in diabetic rat testicular damage and that AG treatment of diabetic rats results in protection of spermatogenic cells against oxidative stress and apoptotic cell death.