Abstract The metabolism of [7- 3H(N)]-testosterone by slices of human lung tissue obtained from an adult male at surgery was studied in a time course experiment. The 17β-hydroxysteroid oxidoreductase activity of human lung was expressed in the synthesis of isotope-labeled 4-androstene-3,17-dione, 5α-androstane-3,17-dione, androsterone and isoandrosterone. The rate of formation of 4-androstene-3, 17-dione followed a linear course for about five min (160 pmol/lOOmg protein/min), while the formation of the other 17-oxosteroid metabolites followed a linear course for about 1 h (5α-androstane-3, 17-dione, 80 pmol/100mg protein/h; androsterone, 2.4pmol/100mg protein/h; isoandrosterone, ∼28 pmol/lOOmg protein/h). Thus, human lung tissue, in vitro, expresses significant 17β-hydroxysteroid oxidoreductase activity and, notably, a cofactor milieu that favors the 17-oxo over the 17β-hydroxysteroid oxidoreduetion state. This obtains since 5α-dihydrotestosterone formation constituted a very small fraction of total 17-oxosteroid metabolites (∼4.6 pmol/100mg protein/h vs. 2,832 pmol/100mg protein/h). In addition to 17β-hydroxysteroid oxidoreductase activity, the human lung has demonstrable 5α-reductase, 3β-hydroxysteroid oxidoreductase and 3α-hydroxysteroid oxidoreductase activities, findings which support the concept that the lung is a site of metabolism of plasma C 19-steroids.