5\g=a\-Dihydrotestosterone (DHT) and 5\g=a\-androstane-3\g=a\, 17\g=b\-diol(3\g=a\-diol) were extracted from epithelium and stroma of human benign prostatic hyperplasia (BPH) and of normal prostate and quantified by RIA. The main results were: (1) concerning the BPH, DHT is mainly located in the nuclear fraction of epithelium and stroma, whereas 3\g=a\-diolwas completely of extranuclear origin, (2) in the nuclei derived from BPH stroma the DHT content (7.1 \ m=+-\0.88 pmol/mg DNA, mean \m=+-\sem [n = 14]) was significantly higher (P < 0.01) than in the nuclei derived from BPH epithelium (3.8 \ m=+-\ 0.38 pmol/ mg DNA [n = 14]). The DHT content in the nuclear fraction derived from unseparated, i.e. whole tissue, was 5.6 \m=+-\0.60 pmol/mg DNA [n = 14], (3) the biological significance of the overwhelming DHT accumulation in the stromal nuclei for the BPH tissue is reflected by a significant correlation between the DHT values in the nuclei from stroma and whole tissue (rs = 0.710, P < 0. 01) and (4) in four normal prostates the DHT content in the nuclear fraction of epithelium (1.3 \m=+-\0.37