Background 5α-Reductase inhibitors (5α-RI) act by inhibiting the conversion of testosterone to dihydrotestosterone (DHT),thereby preventing DHT induced benign prostatic hyperplasia.The existing 5α-RIs can be classified into two types:competitive and noncompetitive.Currently,limited evidence is available concing the effect differences between the two types of 5α-RI on androgens.The purpose of this study was to assess the effects of competitive and noncompetitive 5α-RIs on serum and intra-prostatic androgens in beagle dogs.Methods Twenty beagles with spontaneous benign prostatic hyperplasia were randomly allocated into two groups:epristeride group (n=10) in which beagles were treated with epristeride at 1 mg/kg once a day for 3 months,and finasteride group (n=10) in which beagles were treated with finasteride at 1 mg/kg once a day for 3 months.The levels of intra-prostatic testosterone and DHT were measured before treatment and on day one after three months medication.Serum levels of testosterone and DHT were measured at the same time points.Changes in androgen levels before and after treatment were analyzed,and comparisons were made within each treatment group and between treatment groups.Results After 3-month treatment,serum and intra-prostatic DHT levels all decreased significantly in both the epristeride and finasteride groups.The change of DHT in serum was significantly higher in the finasteride group (-14％ and-43％ in epristeride and finasteride groups respectively,with P＜0.001); however there was no significant difference in the changes of intra-prostatic DHT between the two groups (-47％ and-51％ in epristeride and finasteride groups,respectively,P=0.304).The decreases in DHT levels were accompanied by reciprocal increases in serum and intra-prostatic testosterone levels.Changes of testosterone were significantly higher in finasteride group both in serum (20％ and 42％in epristeride and finasteride groups,respectively,P＜0.001) and in prostate tissue (18％ and 29％ in epristeride and finasteride groups,respectively,P=0.004).Conclusions Two types of 5α-RI have similar effects in reducing DHT in prostate tissue in beagles.Competitive 5α-RI may reduce serum DHT to a greater scale,and significantly increase testosterone in beagle serum and prostate.