Serotonergic mechanisms have been implicated in the pathophysiology of depression and in the neuropharmacology of antidepressant treatment. One measure of central serotonergic function is the prolactin (PRL) response to IV L-tryptophan (L-TRP). We used the L-TRP test to assess the role of serotonin in the mechanism of action of lithium augmentation in refractory major depression. Twenty-six patients with antidepressant-refractory major depression each received three L-TRP tests (after 2 weeks of placebo, after 4 weeks of active primary antidepressant, and after 1 week of lithium augmentation). Primary antidepressant treatment did not increase the PRL response, but lithium augmentation resulted in a statistically significant increase in PRL response as compared to both placebo pretreatment (P less than 0.04) and antidepressant treatment alone (P less than 0.025). This study supports a role for serotonergic mechanisms in the action of lithium augmentation.