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Using isotopic dilution techniques, certain metabolic properties of cholesteryl 3/3-sulfate [cholesterol sulfate (CS)], a substance that has recently been isolated from human blood and adrenal tissue, were studied in two normal human subjects. Thus, the concentration (c) of circulating CS" in the plasma was found to be 328 and 924 jug/100 ml. Following the injection of [14C]CS, the metabolic clearance rate (MCR) of this compound was determined from the disappearance curve of the tracer from plasma. From these data, the half-life was calculated to be 14-15 hr in these subjects. The production rate of cholesterol c X^>holesterol sulfate (CS)* 1 *has recently been shown by Drayer et al. (1964) to be a naturally occurring entity. This conjugate has been found in bovine adrenal tissue, human blood (Drayer et al., 1964), and adrenal tumor tissue (N. M. Drayer, unpublished data). Recently, Moser et al. (1966) also isolated this sulfate from natural sources. That CS may serve, too, as a substrate for the mitochondrial side-chain cleaving enzyme of bovine adrenal tissue has been described by Raggatt et al. (1965). Moreover, Roberts et al. (1964) perfused CS through a human adrenal adenoma, in situ, and the results suggest that CS was converted to a variety of A5-3/3-sulfated steroids via intermediates which retain the sulfate grouping. The purpose of the present study was to determine the plasma concentration of CS, the rate at which blood-borne CS is produced (PRcs), and the quantitative importance of circulating CS as a precursor of steroids. Values for the metabolic clearance rate, MCR, were obtained by injecting 14C-labeled CS intravenously and measuring the concentration of radioactivity present as CS in plasma samples withdrawn at various intervals fol-

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