THE mechanism of action of the lithium ion in man remains obscure although it is being increasingly used especially in the treatment of manic-depressive disorder. In the similarity to sodium in the periodic table, its ability to substitute for sodium in medium surrounding nerves1 and muscles2 in some in vitro systems and its active transport in biological systems3,4 have raised the possibility that a mechanism related, at least in part, to sodium metabolism may explain its actions. Coppen et al initially described several striking changes in electrolyte and water distribution in schizophrenic patients studied before and one week after receiving 1 gm of lithium carbonate daily.5 These changes included an increase in total body water and extracellular fluid (ECF) volume, and a fall in 24-hour exchangeable sodium (Nae), sodium space and residual sodium (Nar). More recently, he was unable to confirm