To investigate the potency of dexamethasone in suppressing adrenocortical function, we gave various evening doses to laboratory staff members. Plasma corticotropin, cortisol, and dehydroepiandrosterone sulfate concentrations were measured the following day, to quantitate the degree of suppression after both short- and long-term treatments. We found that the minimum amount for effective suppression was 1 mg for a single evening dose, 0.5 mg per day for a long-term evening dose. On these regimens plasma corticotropin and cortisol were consistently suppressed, but plasma dehydroepiandrosterone sulfate was suppressed only with long-term treatment.