BACKGROUND: Anabolic-androgenic steroid compounds are one of the most widely abused drugs by athletes and muscle builders with the goal of improving performance/ability, appearance, or muscle mass. In addition, these steroids are widely used in the treatment of male infertility and subfertility. However, increasing concern has been shown that these compounds may not only offer unappreciable benefits to infertile and subfertile males, but might have deleterious effects on both human and animal physiology and sperm quality. There is a dearth of knowledge on the structural and quantitative changes of the testis secondary to this group of compounds. Objective: The present study was carried out to evaluate the effects of mesterolone (proviron), an anabolic-androgenic steroid, on some of the histomorphometric and stereological parameters of the seminiferous tubules in Sprague-Dawley rat. Materials and Methods: Two groups of 10 adult male rats were used. The treated group was given 0.06 mg/kg body weight/day of mesterolone by gavage for six weeks while the control group received equal volume of 0.9% normal saline per day. Five µm of uniformly random serial sections of the processed testicular tissues were analyzed using un-biased stereological and histomorphometric studies. Results:The results showed that the percentage mean volume density of both the tubular lumen and epithelial height increased by 35% (p< 0.05) and decreased by 50% (p<0.05), respectively compared to the control. mesterolone also caused a significant decline in sperm concentration. Conclusion: Mesterolone produces epithelial hypoplasia in the testis post continuous management.