The expression of 5alpha-reductase type 1 and type 2 isoenzymes in hyperplastic human prostate tissue and several human prostate cell lines was investigated by Northern blot analyses, reverse transcription-polymerase chain reaction (RT-PCR), and enzyme activity. Separation of stroma and epithelium was confirmed histologically and only preparations with no apparent contamination were employed in the subsequent studies. Poly(A)+ RNA was isolated from stromal and epithelial fractions and analysed by Northern blot and RT-PCR. Inhibition of epithelial and stromal 5alpha-reductase activities by 17beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5alpha-androstan- 3-one (4MA) was assessed using a range of concentrations between 10(-13) and 10(-5) M. Results from Northern blot analyses and RT-PCR showed that the prostate stroma expressed 5alpha-reductase type 1 and type 2 isoenzymes, whereas the prostate epithelium only expressed 5alpha-reductase type 1. This was consistent with biphasic inhibition of 5alpha-reductase activity by 4MA in stroma and monophasic inhibition in epithelium. Cultured epithelial cells derived from human prostate only expressed 5alpha-reductase type 1 and had Vmax and Km values that approximated the lower end of the range reported for surgically removed prostate epithelium. The foregoing data explains the disparate activities of 5alpha-reductase, previously reported, in stroma and epithelium. The differential localization of these isoenzymes in the prostate suggests that future therapy of androgen-sensitive disease may be more successful through the use of selective inhibitors of the different 5alpha-reductase isoenzymes.