The therapeutic efficacy of lithium added to tricyclic antidepressants (TCA) or related thymoleptics was investigated in 27 endogenous depressive patients responding unsatisfactorily to these drugs alone. All patients received lithium during two weeks, however, with a delay of one week in 13 of them, the attribution to active treatment or placebo for the first seven days being double-blind. A statistically significant improvement compared to placebo occurred after lithium addition. The results of the double-blind part of the study as well as the observations during the total two-week period suggest that endogenous depression failing to respond to thymoleptic drug treatment can be improved within a short time by the co-administration of the two treatments in the indicated sequence. However, the proportion of patients improved and the rapidity of improvement noted in this study were less favorable than reported by the investigators who discovered the therapeutic principle.