Cortisol and corticosterone productions were measured by an isotope dilution technique described by Cope and Black in 1958. Provided that some minor modifications are introduced which increase the specificity, this method is very valuable for measuring corticoid hormone secretion rates. Cortisol production in normal men varied between 15.9 mg and 30.9 mg per 24 h with a mean of 21.4 mg; corticosterone production varied between 1.6 mg and 5.5 mg with a mean of 3.4 mg. The mean F./B production ratio was 6.7. In a few patients with adrenal hypo- and hyperfunction, production rates were determined by the same technique.