Sex steroid concentrations and 17β-hydroxysteroid dehydrogenase andaromatase activities were determined in fat tissue removed at surgery or, in order to allow comparisons in different sites, postmortem. Except for dehydroepiandrosterone (DHEA) sulfate (DHEAS), there existed a positive tissue/plasma gradient for all steroids studied (testosterone, androstenedione, DHEA, androstenediol, estrone, and estradiol), suggesting androgen uptake and estrogen synthesis in situ Androgen concentrations did not vary according to site of origin of fat tissue, except that the DHEAS concentration was significantly lower in abdominal sc and omental fat than in breast, pericardial, or sc pubic fat. Tissue androgen concentrations were positively correlated with their plasma concentrations, but tissue and plasma estrogen concentrations were not correlated. All tissue steroid concentrations, with the exception of estradiol in men, decreased with age. Aromatase activity [androstenedione −*· estrone; mean maximum velocity, 7.4 ± 3.7 (±SD) fmol estrone/ mg protein · h] did not vary between sexes or with site of origin of fat tissue. 17β-Hydroxysteroid dehydrogenase activity (estradiol — estrone, mean maximum velocity 9.8 ± 5.4 pmol/mg protein · h) was higher in fat from women than in that from men, higher in premenopausal than in postmenopausal women, and highe in omental tha in sc fat. Its activity was noncompetitively inhibited in vitro by DHEA and DHEA in near-physiological concentrations, and the enzyme activity was inversely correlated (P < 0.001) with the tissue DHEA and DHEAS concentrations.We conclude that fat tissue is an important steroid hormone reservoir, that it i the site of active aromatase and 17β-hydroxysteroid dehydrogenase, and that tissue DHEA(S) may have a modulating effect on tissue estrogen production.