OBJECTIVE The oxidative and reductive stresses within the varicocele veins were estimated. Nitric oxide synthase and xanthine oxidase activities, as well as nitric oxide, S-nitrosothiols and superoxide release within the spermatic vein in patients with varicocele, and the role of the noxious oxidant peroxynitrite formed from nitric oxide and superoxide in sperm dysfunction were determined. METHODS Whole blood samples were drawn from a peripheral vein and a dilated varicocele vein before ligation. Nitric oxide synthase, xanthine oxidase, nitric oxide and peroxynitrite were measured by novel spectro-fluorophotometric methods. S-nitrosothiols were estimated by a luminol-chemiluminescence method. Serum and red blood cell antioxidant capacity was determined by a chemiluminescence reaction. RESULTS Serum nitric oxide synthase and xanthine oxidase activities, as well as nitric oxide, peroxynitrite and S-nitrosothiol levels were greater in the spermatic vein compared to the peripheral vein. Serum antioxidant capacity was greater in varicocele veins compared to peripheral veins. In contrast, the antioxidant capacity of red blood cells was less in the varicocele veins, which was consistent with an increased rate of peroxynitrite production. CONCLUSIONS Our data suggest a high oxidative stress due to the release of nitric oxide synthase and xanthine oxidase within the dilated spermatic vein. The reaction resulted in dramatic formation of nitric oxide, peroxynitrite and S-nitrosothiols, which are biologically active. Formation of peroxynitrite from the reaction of nitric oxide with superoxide could be a causative factor for impaired sperm function in patients with varicocele.