Background: Nandrolone decanoate (ND) is an anabolic androgenic steroid (AAS) which influences the ovarian structure and function. We assessed the effects of ND on the ovarian volume, number of primordial follicles, and level of hormones and also evaluated the modulatory effects of gonadotropins on the histopathological changes imposed by the administration of ND. Methods: Six groups of Sprague-Dawley adult female rats (n=30) were used. The experimental rats were injected intraperitoneally with 3 and 10 mg/kg ND with or without human menopausal gonadotropin (hMG), 10 IU weekly for one month. The vehicle and control rats were administered olive oil and saline, respectively, for the same period of time. The ovarian volume and number of primordial follicles were estimated by stereological methods. Results: The results showed a decrease in the ovarian volume, number of primordial follicles, and level of gonadotropins in the ND-treated animals compared with the vehicle groups. In the rats treated with 3 mg/kg of ND with hMG, an increase in the ovarian volume and number of primordial follicles was shown as compared to the rats treated with the same dose of ND without hMG. Conclusion: ND exerted detrimental effects on the dimensions of the ovary, number of follicles, and level of sex hormones. However, hMG, prevented the harmful effects of ND (at least in a low dose) on the ovarian follicles.