The effects of osaterone acetate (OSA), which is an anti-androgen agent being developed as a therapeutic drug for benign prostatic hypertrophy (BPH) in dogs, on the degree of prostatic regression and semen qualities were investigated. Prostatic regression was compared between dogs with and without orchidectomy. Five male beagles aged 5-9 years were used in the experiment. OSA was orally administered at doses of 0.2 mg/kg and 0.5 mg/kg for one week. The prostatic regression rate one week after the end of administration was 62.6% on average. In the orchidectomized group, the mean regression rate one week after orchidectomy was 60.1%. However, the prostate became enlarged 6 months after administration, compared to the size prior to administration. The above findings suggested that OSA is clinically applicable as a therapeutic drug for BPH in dogs, and inhibits prostatic hypertrophy during the early phase.