An in vitro system for testing steroids which might be effective in treating benign prostatic hypertrophy (BPH) has been developed based upon the transformation of H-3-testosterone into the 5-alpha-reduction products dihydrotestosterone and 3-alpha-androstanediol. In scrutinizing the influence of the amount of BPH-tissue, time, and pH, 300 mg of tissue incubated for 2 h at the physiological pH of 7.4 were used in the standard experiment. -The H-3-testosterone concentration was varied from 0.17-100 times 10-8 M. Plotting the resulting 5-alpha-reduction products as a function of testosterone concentration a hyperbolic pattern of enzyme kinetics ensued. Performing a double reciprocal plot of 4 experiments with double determination of each value regression lines could be computed. Those two regression lines most different in their slopes were considered "normal" limits. The rate of H-3-testosterone metabolism could not be enhanced after the endogenous testosterone content within the prostate glands had been used up by means of a preincubation. Scrutinizing the effect of heparin, a weak non-specific enzyme inhibitor, no suppression of the appearance of 5-alpha-reduction products was found. Damaging the BPH-cells, however, by repetitive freezing and thawing lead to an almost complete inhibition of H-3-testosterone turnover.