Recently, boldenone (androgenic steroid) is used in improvement of the growth and food conversion in food-producing animals. In addition, it is used by bodybuilders during both off-season and precontest, where it is well known for increasing vascularity while preparing for a bodybuilding contest. The present study was designed to investigate the possible effect of growth promoter boldenone undecylenate on the structure and functions of rabbit testes. A total of 32 adult New Zealand rabbits were divided into 4 groups. The first group in the control group includes animals that were intramuscularly injected with olive oil and dissected after 3 weeks. Three experimental groups include animals that receive 1, 2, and 3 intramuscular injections of 5 mg/kg body weight boldenone, and dissected after 3, 6, and 9 weeks, respectively. Treating rabbits with boldenone increased the testosterone levels compared to the control group. Seminiferous tubules of the rabbit testis treated with boldenone showed reduced development and degeneration of the germinal epithelium, leading to debris and syncytial cell formation in the lumina of seminiferous tubules. Our immunohistochemical results indicated severe reduction in proliferating cell nuclear antigen-positive spermatogonia in boldenone-treated animals as compared to the control group. These findings explain the common phenomena among athletics and bodybuilders who suffer from infertility as they were injected with some drugs such as steroids (boldenone) to build muscles.