Lithium ion provides a useful and specific form of chemotherapy for manic and hypomanic episodes, although its antimanic effect may be delayed for as long as a week or more, requiring the use of an antipsychotic agent in the initial period to control the behavior of very disturbed patients. The mechanisms of action of lithium remain obscure, but probably involve effects on neuronal and hormone-target cell membranes. Lithium has interesting antithyroid and anti-antidiuretic hormone effects that are potentially useful medically. The main limitation of the use of lithium is its narrow therapeutic index and requirement of close medical supervision. The most promising aspects of the use of lithium are its encouragement of better psychiatric diagnosis and its "prophylactic" effectiveness in at least reducing the frequency and severity of manic and depressive attacks in manin-depressive illness.