The clinical efficacy of lithium augmentation in refractory depression is hypothesized to depend on the ability of lithium to enhance presynaptic 5-hydroxytryptamine (5-HT) function. Since fenfluramine promotes release and inhibits reuptake of presynaptic 5-HT, we assessed its efficacy in augmenting ongoing tricyclic antidepressant treatment of refractory depression. Fifteen patients with DSM-III major depression failed to respond to treatment with desipramine 2.5 mg/kg/day or more (plasma levels of at least 125 ng/ml) given for at least 4 weeks. Fenfluramine 40-120 mg/day was then added to the ongoing desipramine in a placebo substitution design. There was no statistically significant evidence of either transient or sustained clinical improvement during the 2 weeks of fenfluramine augmentation. One patient appeared to respond to the treatment, but one appeared to worsen. Fenfluramine more than doubled steady-state plasma levels of desipramine. These findings suggest that lithium's efficacy as an augmenting agent depends on properties that are not shared by fenfluramine. Fenfluramine cannot be recommended in the routine management of refractory depression.