Studies on plasma aldosterone, total serum protein, electrolytes, osmolal concentrations in serum and urine, creatinine, lithium and osmolal clearances were carried out on 14 patients suffering from lithium intoxication. The determinations were done on samples obtained on admission and during the following twelve days of treatment with hemodialysis, sodium chloride loading, or forced diuresis. Plasma aldosterone and total serum protein were increased; serum sodium concentrations and creatinine clearances were decreased on admission. Serum osmolality was normal; urine osmolal concentration was just above that in plasma. The results showed that the lithium intoxicated patients were moderately depleted of sodium and water on admission. Plasma aldosterone, total serum protein and serum sodium were normalized during treatment. Creatinine clearance increased but did not normalize in all patients. Renal concentrating tests were performed in five patients three to four weeks after lithium intoxication. The renal concentrating ability was reduced in all. The elevated plasma aldosterone seen in lithium intoxication might have been a result of sodium and water depletion. It is suggested that impaired renal concentrating ability induced by lithium may make some patients more susceptible to conditions which can lead to sodium and water loss, and thereby to lithium intoxication.