Preliminary reports suggested that the addition of lithium carbonate to the regimen of patients treated with, but not responding to, a tricyclic antidepressant (TCA) drug can induce a rapid alleviation of depression. We examined the effect of lithium carbonate addition in 39 patients with unipolar depression whose conditions were not improved by at least three weeks' TCA drug administration. In 30 of 42 observations, lithium carbonate brought about a greater than 50% improvement within 48 hours. In a second study, the effects of lithium carbonate addition were compared in five amitriptyline hydrochloride-pretreated and five placebo-pretreated patients who showed no improvement after a three-week treatment. All five patients receiving amitriptyline showed a greater than 50% improvement 48 hours after lithium carbonate addition, whereas only one patient in the placebo group showed a marked response. In a third study the effect of lithium carbonate withdrawal was studied in nine TCA-resistant patients who had shown a marked improvement 48 hours after lithium addition. Only five of these patients had a relapse five days after lithium discontinuation. Since animal studies have shown that TCA drugs sensitize forebrain neurons to serotonin and that lithium enhances the activity of serotonin-containing neurons, we propose that the antidepressant effect of lithium addition in TCA-resistant patients might be mediated by enhancing serotonin neurotransmission.