The present study reports the effects exerted by 4-hydroxy-4-androstene-3,17-dione (4-OH-A) on the in vitro metabolism of labelled testosterone, dihydrotestosterone (DHT) and androstenedione (delta-4-A) in the prostate of adult male rats and in human benign prostatic hypertrophic (BPH) tissue. It has been found that 4-OH-A decreases the formation of DHT and of the diols. When testosterone is used as the substrate, the presence in the medium of 4-OH-A enhances the formation of delta-4-A and of 5-alpha-androstane-dione (5-alpha-A); 4-OH-A does not inhibit the conversion of labelled DHT into the diols. Also, the transformation of labelled delta-4-A into 5-alpha-A is not modified by 4-OH-A. On the basis of these findings, it is suggested that 4-OH-A might represent a potential new agent for the prevention and/or treatment of human BPH.