The present article reviews the history of research on the hydroxylation of steroid hormones as catalyzed by enzymes present in mammalian tissues. The report describes how studies of steroid hormone synthesis have played a central role in the discovery of the monooxygenase functions of the cytochrome P450s. Studies of steroid hydroxylation reactions can be credited with showing that: (a) the adrenal mitochondrial enzyme catalyzing the 11 beta-hydroxylation of deoxycorticosterone was the first mammalian enzyme shown by 0 18 studies to be an oxygenase; (b) the adrenal microsomal enzyme catalyzing the 21-hydroxylation of steroids was the first mammalian enzyme to show experimentally the proposed 1:1:1 stoichiometry (substrate: oxygen: reduced pyridine nucleotide) of a monooxygenase reaction; (c) application of the photochemical action spectrum technique for reversal of carbon monoxide inhibition of the 21-hydroxylation of 17 alpha-OH progesterone was the first demonstration that cytochrome P450 was an oxygenase; (d) spectrophotometric studies of the binding of 17 alpha-OH progesterone to bovine adrenal microsomal P450 revealed the first step in the cyclic reaction scheme of P450, as it catalyzes the "activation" of oxygen in a monooxygenase reaction; (e) purified adrenodoxin was shown to function as an electron transport component of the adrenal mitochondrial monooxygenase system required for the activity of the 11 beta-hydroxylase reaction. Adrenodoxin was the first iron-sulfur protein isolated and purified from mammalian tissues and the first soluble protein identified as a reductase of a P450; (f) fractionation of adrenal mitochondrial P450 and incubation with adrenodoxin and a cytosolic (flavoprotein) fraction were the first demonstration of the reconstitution of a mammalian P450 monooxygenase reaction. (c) 2005 Elsevier Inc. All rights reserved.