In man, unconjugated plasma DA is normally undetectable or present in minute amounts. Twelve medication-free volunteers received a 1 mg dose of dexamethasone which produced pronounced increases of plasma free DA but not of other catecholamines. Mean plasma free dopamine levels after dexamethasone at 8 a.m. (155 +/- 102 pg/ml) and 4 p.m. (163 +/- 70 pg/ml) were significantly higher (p less than 0.001) than those at 8 a.m. (50 +/- 18 pg/ml) and 4 p.m. (42 +/- 7 pg/ml) before dexamethasone. Although the mechanism of increased dopaminergic activity after a dose of dexamethasone remains for future research, the data presented in this paper may explain the observations that corticosteroids lower prolactin levels and may induce psychiatric disturbances, as well as the finding that depressed patients with high postdexamethasone cortisol levels are frequently psychotic.