Eight beagles with benign prostatic hypertrophy (BPH) were treated by subcutaneous implantation of pellets containing 10 mg/kg chlormadinone acetate (CMA), a synthetic anti-androgen, plus daily oral administration of CMA at 2 mg/kg per day for 7 days as a therapy for BPH. Prostatic and testicular size were measured and prostatic and testicular biopsies were performed by laparotomy before and after CMA treatment. Plasma levels of luteininzing hormone (LH), testosterone and oestradiol were also measured. The clinical signs of BPH, for example haematuria and dysuria, resolved within 1 week of treatment. Mean prostatic volume decreased to 56% of the pretreatment value. At 40 weeks after treatment, prostatic volume had decreased by 36%. Histological examination of the prostate 1 week after treatment revealed reduction in diameter of the alveoli and in height of the glandular epithelium. Degeneration and atrophy of the glands were marked 4-12 weeks after treatment. In the testis, the diameter of seminiferous tubules and the number of germ cells in the seminiferous tubules had decreased markedly at 12 and 24 weeks after treatment. Although plasma LH concentrations did not undergo any marked fluctuations after CMA treatment, levels of testosterone and oestradiol were lower than before treatment. The results indicate that implantation of 10 mg/kg CMA, plus 7-day oral administration of 2 mg/kg CMA, bring about resolution of the clinical signs and marked reduction in prostatic volume within 1 week of treatment.