The oxidative and reductive biotransformations of 9 alpha-fluorocortisol (fluorocortisol) by human liver microsomes and cytosol have been characterized. 9 alpha-Fluorination greatly simplified cortisol metabolism in microsomes: dehydrogenation of the 11 beta-hydroxyl group and A-ring (4-ene-5 beta and 3 alpha-keto) reduction, the principle pathways, were completely blocked. Fluorocortisol was essentially metabolized by the remaining pathways, 20 beta-reduction and 6 beta-hydroxylation. In cytosol, 20 beta-reduction replaced the A-ring reduction of cortisol as the sole biotransformation. The major structure-metabolism relationships of fluorocortisol in man, i.e. complete and extensive inhibition of 11 beta-dehydrogenation and 4-ene-5 beta-reduction, respectively, were attributed to hepatic enzyme systems. Their mechanistic basis is discussed with reference to the electronic and conformational changes induced by 9 alpha-fluorination.