Preclinical evidence suggests that alterations in serotonin function may relate to the development of anxiety and the therapeutic effectiveness of antianxiety treatments. Serotonin increases prolactin release, and intravenous administration of the serotonin precursor, tryptophan, produces reliable elevations in serum prolactin levels. To evaluate serotonergic function, the effects of intravenous tryptophan on prolactin secretion were determined in 23 drug-free patients meeting DSM-III criteria for agoraphobia with panic attacks or panic disorder and 21 age- and sex-matched healthy subjects. In nine of the patients the tryptophan infusion was repeated during long-term alprazolam treatment. The ability of tryptophan to increase prolactin levels was not different between the patients and healthy subjects and was not altered by alprazolam treatment. These findings suggest serotonin function may be normal in panic anxiety disorders and the antipanic mechanism of action of alprazolam may be unrelated to effects on serotonin activity.