It is conceivable that androstenedione contributes indirectly to 5alpha-dihydrotestosterone formation in human prostate by its intraprostatic conversion to testosterone. This reversible conversion is catalyzed by the enzyme 17beta-hydroxysteroid oxidoreductase (17beta-HSOR). At present, rather limited information on kinetic parameters like specific concentration (V(max)), affinity to steroid substrates (K(m)S) and to pyridine nucleotides (K(m)N) of 17beta-HSOR is available, Thus, we determined those aforementioned kinetic parameters in epithelium and stroma of normal human prostate (NPR) and benign prostatic hyperplasia (BPH). The main results were: (1) the mean K(m)S of 17beta-HSOR(red)/NADPH was significantly (P < 0.0001) lower than those of all other 17beta-HSORs. (2) In almost all cases the mean V(max) was higher in BPH than NPR. (3) In all cases, the mean V(max)/K(m)S ratios of 17beta-HSOR(red) were higher than those of 17beta-HSOR(ox). The highest ratio was found regarding 17beta-HSOR(red)/NADPH in BPH stroma. (4) In stroma, a significantly positive correlation of V(max)/K(m)S of 17beta-HSOR(red)/NADPH with age was found. (5) The lowest K(m)N was found regarding NADP+, followed by NADPH. It is concluded that in human prostate the balance of the reversible conversion of testosterone to androstenedione is shifted potentially towards testosterone, particularly in BPH stroma.