Dihydrotestosterone (DHT) is the principle androgen in certain tissues such as the prostate. DHT is formed from testosterone by the NADPH-dependent enzyme 5 alpha-reductase (5AR). In this paper we report the expression of catalytically active steroid 5AR from the rat in Saccharomyces cerevisiae. A full length cDNA coding for 5AR was isolated from a rat liver cDNA library and fixed in frame to the signal sequence of yeast acid phosphatase. A constitutive short promoter fragment of the acid phosphatase gene (PHO5) and the PHO5 transcriptional terminator were added and the expression cassette ligated into the yeast 2 mu vector pDP34. S. cerevisiae transformed with the 5AR expression plasmid pDP34/PHO5AR exhibited about 100-fold more activity per gram wet weight than rat prostate.