A study of 60 healthy volunteers ranging from 19-88 yr of age found nonsuppression rare (5%) and confirmed that the dexamethasone suppression test (DST) of the reactivity of the hypophyseal-pituitary-adrenal axis to negative feedback inhibition requires 0800 h plasma dexamethasone (DEX) levels in excess of 220 ng/dl. All of the subjects with inadequate DEX levels (N = 3) were older than 50 yr of age as were the nonsuppressors (N = 3); two of the three nonsuppressors had inadequate DEX concentrations. Thus, DST may be more often invalid in elders than in younger adults because of inadequate plasma dexamethasone (DEX) concentration, indicating that plasma DEX levels should be assayed concomitantly with DST in elders.