Impaired suppression of plasma cortisol levels following a single midnight dose of dexamethasone has been well documented in a large subgroup of depressed patients. To interpret meaningfully this 24-hr dexamethasone suppression test in geriatric patients, the effect of age per se was examined by comparing the pattern and duration of suppression in ten healthy elderly and ten healthy young subjects. Advanced age did not appear to affect the overnight dexamethasone suppression in healthy humans and should not invalidate the use of dexamethasone suppression test as a laboratory tool in the diagnosis and investigation of depression.