The hormonal response to the serotonin releasing agent/uptake inhibitor fenfluramine has been used as an indicator of central serotonin system function. The serotonergic system plays an important role in the etiology and pathogenesis of mood disorders. We compared the prolactin response to fenfluramine administration in unipolar depressed patients (major depressive disorder), depressed patients with bipolar disorder, and healthy controls. We found a trend towards a blunted prolactin response in depressed patients compared to healthy controls, after controlling for sex, family history, family history-by-gender interaction, and baseline levels. There was no significant difference between unipolar and bipolar patients in the baseline prolactin levels or the response to the fenfluramine administration. We also found a negative correlation between aggression and impulsivity scores and prolactin responses in subgroup with unipolar but not bipolar depression. Female patients with unipolar depression who had first-degree relatives with unipolar depression and normal controls had significantly higher prolactin responses than female patients with unipolar depression who did not have first-degree relatives with unipolar depression. The lack of difference in the response to fenfluramine administration between unipolar and bipolar depressed patients may indicate that overall serotonergic function in unipolar and bipolar depressed patients is similarly impaired.