In order to elucidate at which level glucocorticoids act in inhibiting thyrotropin (HTSH) secretion in man, 200µgm of synthetic thyrotropin releasing hormone (TRH) were injected intravenously in 8 normal subjects before and after oral administration of dexamethasone, 1.0 mg every 8 hr for 5 consecutive days. After dexamethasone, the mean basal plasma HTSH levels were significantly reduced, as was the plasma HTSH response to TRH: this reduced increase occurred in all subjects. The glucocorticoid did not alter significantly the serum thyroxine and triiodothyronine uptake. The data obtained are compatible with a direct inhibiting action of dexamethasone at pituitary level.