Testicular tissue from 6 patients (ages 24–84 yr) was incubated with 14C-testosterone (17β-hydroxy-4-androsten-3-one), and the conversion to 5α-dihydrotestosterone (17β-hydroxy-5α-androstan-3-one) was measured. All testicular samples showed a small but consistent conversion to 5α-dihydrotestosterone. This study demonstrates the presence of the Δ4-3-ketosteroid 5α-reductase enzyme in the adult human testis.