Abstract Objective The aim of the present investigation was to characterize the effects of supraphysiological doses of the anabolic androgenic steroid nandrolone decanoate (ND) on the fertility of female rats, as well as on the morphology of their uterus. Study design Female Wistar rats ( n = 15) received a subcutaneous injection of ND (15 mg/kg) once daily during a 2-week period, while the control animals ( n = 10) were administered vehicle alone (arachidis oleum) in the same manner. Estrus behavior was evaluated 4 weeks after termination of this treatment and in cases where signs of receptivity were present, the female rat was given the opportunity to copulate with a male. After breeding, the female animals were sacrificed and their uteri examined histomorphologically. Results All ND-treated animals exhibited abnormal vaginal smears, whereas all of the control smears were normal. Most (73%) of the treated females demonstrated normal estrus behavior (i.e., willingness) on the day of mating, but none got pregnant; whereas all of the control rats became pregnant. The female rats receiving the ND showed an enhanced rate of weight gain and the myometrium thickness of their uteri was significantly increased, while the endometrium was significantly thinner. Furthermore, ND caused a significant proportion of the treated animals to display tortuous and irregularly branching endometrial glands, as well as a lack of the physiologically normal infiltration of eosinophilic leukocytes into the endometrium (endometrial eosinophilic homing), a finding that has not been reported previously. Conclusion The present findings indicate that high doses of ND cause morphological and physiological alterations in the uterus of female rats that are associated with a suppression of their reproductive capacity.