Objective: Increased nitric oxide in the spermatic veins of men affected by varicocele has been reported. NO plays an important role in the varicocele-induced decrease of seminal quality. iNOS activity may play a role in the testicular dysfunction associated with varicocele during adolescence. iNOS activity was slightly increased in the leydig cells of varicocelized rats. It is a clinical case that inhibitors of NOS have succeeded in enhancing motility. Aminoguanidine is an inhibitor of iNOS. The objective of this study was to investigate the role of AG in efficiency of sperm parameters in varicocelized rat. Materials and Methods: Twenty four wistar male rats divided into four groups. The group A and B underwent a left experimental varicocele. In group C (sham group) rats underwent a similar procedure but without any change on spermatic vein. Group D referred to as control. Animal in group A were killed 10 weeks after the operation. Epididymal sperm parameters were evaluated. Group B received 50 mg/kg AG intraperitoneally daily for ten weeks. Results: Sperm parameters decreased significantly in the group A and the group B in comparison with group C and D (p≤0.05). The sperm parameters increased in the group B that received AG in comparison with the group A (p≤0.01). Group C (sham) did not show any significant alteration in sperm parameters in comparison with the control group. Conclusion: These findings suggest that iNOS is an important mediator in the pathogenesis of varicocele. This study may support the concept that the use of iNOS inhibitor in infertile men with varicocele may be useful.