Alterations in serotonin metabolism may be an important factor in the etiology and treatment of depression. In this regard, 5-hydroxytryptophan (5-HTP), a serotonin precursor, has been given to patients with depression. Although a review of these studies suggests that 5-HTP possesses antidepressant properties, additional trials are clearly indicated. Following a discussion of the pharmacology of 5-HTP, the authors highlight adverse effects associated with its administration to depressed patients, neurologic subjects, and normal individuals. Relatively few adverse effects are associated with its use in the treatment of depressed patients.