In 17 depressed patients with initially abnormal results on the dexamethasone suppression test (DST), serial plasma samples for the determination of cortisol concentrations were taken every 10 days, following overnight dexamethasone administration at 11 p.m. Severity ratings were repeated on the days of blood sampling. There was a gradual normalization of the DST and progressive clinical improvement during selective antidepressant therapy. The DST was closely related (r = 0.573, P less than 0.005) to the patients' clinical mood level during the depressive episode. At the point where normalization of the DST occurred, the patients were still moderately severely ill. DST conversion occurred early in the treatment, i.e. after 23.9 (+/- 15.1) days, and preceded symptomatic improvement by 24.5 (+/- 18.1) days. Normalization of the DST was a predictor (r = 0.691, P less than 0.005) of the time of clinical improvement, but not of clinical recovery. The test was a biological discriminator between severe and less severe depressions. The time of symptomatic improvement (r = 0.505, P less than 0.05), but not of biological remission, depended on age; severe depressions lasted longer in the elderly patients.