The effects of a pure glucocorticoid, dexamethasone, and a pure mineralocorticoid, desoxycorticosterone, on the serum concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3), and 3,3',5'-triiodothyronine (reverse T3, rT3) were compared both in healthy subjects and in athyreotic T4-substituted patients. In addition, the effect of exogenous ACTH was examined in healthy subjects. Both in healthy subjects and in T4-substituted athyreotic patients, administration of a single oral dose of dexamethasone caused a rapid and sharp decrease in the serum concentration of T3, and a corresponding increase in the serum concentration of rT3. The T4 concentration was not changed. A single oral dose of desoxycorticosterone evoked no significant changes in the serum concentrations of T3, rT3, or T4 either in healthy subjects or in T4-substituted athyreotic patients. Like dexamethasone, ACTH (two i.v. injections of 60 IU each, at a 6-hour interval) evoked a serum T3 reduction and a serum rT3 increase. Hence, it appears that both endogenous and exogenous glucocorticoids, but not mineralocorticoids, may partially divert the deiodination of T4 from the activating (T4 lead to T3) to the inactivating (T4 leads to rT3) pathway.