The efficacy of lithium in the acute manic phase of manic-depressive disorder is well established. 1 There is also evidence indicating a prophylactic effect for this ion in manic-depressive disorder. 2 The efficacy of lithium carbonate in the acute depressive phase is uncertain. Early observations tended to discount any antidepressant effect of lithium; 3 however, recent evidence derived from controlled trials, although conflicting, tends to suggest that an antidepressant effect may occur in some patients. 4,5 Fieve, 6 in comparing lithium and imipramine response, concluded that the antidepressant effect of lithium was mild. Dyson and Mendels, 7 first in an uncontrolled study and later in a controlled study comparing lithium with desipramine, 5 concluded in the later study that lithium showed marked antidepressant properties equivalent to desipramine. As part of our investigative program with lithium, we undertook a single-blind evaluation of the efficacy of lithium carbonate in depressive illness.