Purpose of investigation: To evaluate the effect of buserelin acetate on the morphology of the endometrium of adult, non-castrated, female Wistar rats. Methods: Female Wistar rats at estrus or diestrus (assessed by vaginal cytology) received daily subcutaneous injections of 20 mg buserelin acetate for four, eight or 12 days. Rats were sacrificed 24 hours or five days following final dosage. A control group received diluent for 12 days. Results: Progressive tissue hypotrophy occurred during treatment and was followed by estrogenic hyperactivity five days after the end of treatment. Vaginal cytology and endometrial histology revealed intense, vacuolized lining and glandular epithelia, brush borders and endometrial stroma densely infiltrated with eosinophils. Conclusions: Buserelin acetate appears to cause a progressive blockade of gonadotrophin secretion when administered to female rats for four, eight or 12 days, and an important rebound effect, with accentuated estrogen release already apparent in the first estrous cycle following treatment.