This experiment was carried out to investigate the effect of endogenous nitric oxide (NO) on the ischemia-reperfusion injury of testis. Testicular ischemia was achieved by twisting the right testis and spermatic cord 1080 counter-clockwise for 30 minutes and reperfusion was allowed for 30 minutes after detorsion of 33 rats. Animals were treated with normal saline in controls just before detorsion, NG-nitro-L-arginine methyl ester (L-NAME), and L-arginine (L-arg) in others. The tissue damage was evaluated with light microscopy, malondialdehyde (MDA) level in tissue, and the blood flow measurement using 133xenon (Xe) clearence technique. MDA indicator of reperfusion injury increased 25% after detorsion when only normal saline was given, L-NAME further increased MDA, L-arginin decreased MDA to control level. L-arginin infusion during the detorsion reduced the reperfusion injury of testis and improved the testicular blood flow after the detorsion.