Varicocele models can be induced in rats by partially ligating the left renal vein. Induction of left varicocele in Wistar rat results in a bilateral increase in testicular temperature and a bilateral decrease in epididymal sperm content and motility. The above detrimental effects of left varicocele on bilateral testicular spermatogenesis and epididymal sperm maturation process can be counteracted by the administration of factors inducing Abstract of the 8th Royan International Twin Congress, Tehran, Iran, 5-7 September 2007 22 Yakhteh Medical Journal, Vol 9, Sup 1, Summer 2007 testicular vascularization. The administration of indomethasin or hCG to varicocelized rats improves bilateral testicular function. Induction of left varicocele in rabbits has shown that there is a defect in glucose metabolism in the varicocelized testis. This experiment tends to suggest that varicocele is a metabolic disease. Further experiments in varicocelized rats or rabbits have shown that left varicocele affects detrimentally the oxidative status of DNA in sperm nucleus. This may be the reason that embryos developed from the fertilization of oocytes with spermatozoa recovered from varicocelized animals have a lower potential for early development and implantation. Performance of varicocelectomy in experimental animals with left varicocele can ameliorate the detrimental consequences of left varicocele on bilateral spermatogenesis, epididymal sperm maturation, and sperm nucleus oxidative status.