The possible role of THF and THE as intermediates in the biotransformation of cortisol to the cortoic acids was studied by giving 3H-THF + 14C-cortisol tracers and 3H-THE + 14C cortisol tracers to two subjects each, measuring the 3H/14C isotope ratios of the urinary cortoic acid metabolites and relating these ratios to the dose ratio. Isotope ratios substantially higher than the dose ratio indicate that the tetrahydro compound is a better precursor than cortisol, and isotope ratios that are essentially identical to that of urinary THF or THE, respectively, indicate that the tetrahydro compound may be an obligatory intermediate in the cortisol leads to cortoic acids pathway. The isotope ratio data in these studies clearly establish that THF was a preferential precurosor of the 11 beta-hydroxy cortoic acids (cortolic and beta-cortolic) and THE was a preferential precursor of the 11-ketone cortoic acids (cortolonic and beta-cortolonic). Furthermore, the data strongly suggest that THF and THE may be obligatory intermediates in cortoic acid formation.