OBJECTIVE The effects of mepartricin (S-160) on spontaneous canine benign prostatic hyperplasia (BPH) were investigated by histological, histochemical and biochemical analysis. METHODS Aged beagle dogs (5-9 years old) with spontaneously developed BPH were treated orally with a placebo or S-160 (5, 10 or 20 mg/kg/day) for 8 weeks. The methodology included measurement of prostatic volume by transrectal ultrasonography, qualitative evaluation of prostatic morphology, determination of plasma and intraprostatic estradiol level by radioimmunoassay and immunohistochemical detection of estrogen receptors and androgen receptors in the prostate. RESULTS S-160 significantly reduced the prostatic volume and regressed histologically the hyperplastic grade of prostate, and also fairly decreased the plasma and intraprostatic estradiol concentration and the estrogen and androgen receptors in the prostate. CONCLUSIONS These results suggest that the reduction of estradiol and estrogen receptors in the prostate may play a crucial role in the regression of BPH by S-160.