Abstract: A convenient and rapid model in vitro to screen steroid 5α-reductase inhibitors which were effective in the treatment of benign prostatic hyperplasia （BPH） was developed. In the attendance of nicotinamide adenine dinucleotide phosphare （NADPH）, steroid 5α-reductase converts testosterone to dihydrotestosterone which is a major etio-logic factor of BPH.NADPH has characteristic absorbance at 340nmol/L，and the absorbance spectrum may be used to identify NADPH as a kind of the substrate in this enzymatic reaction. In this paper, NADPH, steroid 5α-reductase，series concentration of testosterone and finasteride, and 4 ml Tris-HCl buffer were continuously incubated together at 37°C and the NADPH OD value were continually measured.The descending rate of NADPH OD340 nm value by linear regression from the beginning to the tenth minute is close to the initial velocity of the enzymatic reaction. The precise activity of the steroid 5a-reductase was the slope after subtracted the one of the blank control. The inhibition constant（Ki）of steroid 5α-reductase inhibitors could be calculated according to the Lineweaver-Burk plots. Two drug screen models, the common isotope model and this novel model，were compared in this paper. The result showed that the latter one is more economical, quicker and more effective than the former one.

Key words: Steroid 5α-reductase inhibitors, Nicotinamide adenine dinucleotide phosphate （NADPH）, Spectrophotometry, Drug screen model, Benign prostate hyper-plasia）, Epristeride