The daily rate of production of cortisol in humans was determined by a new method and the resulting values were found to be similar to those obtained by a standard method. The new method entails the injection of a known dose of 3H-cortisol into the subjects, followed by collection of urine for one or two days. Those metabolites having the 3α-hydroxy-5β-pregnane configuration were chemically converted to 11β-hydroxyetiocholanolone, which was then isolated, crystallized and its specific activity determined by counting a weighed sample. The daily rate of production of cortisol was calculated from the dose and from the specific activity of derived 11β-hydroxyetiocholanolone. This procedure avoids the use of both enzymatic hydrolysis of the conjugated metabolites and colorimetry. The new method is more rapid, more economical and more precise than existing methods.