Changes in urinary $Na^+$ and $K^+$ excretions, renal cortical microsomal $Na^+$ -K-ATPase activity, cortical tissue electrolyte content and plasma aldosterone level were studied in rats treated with CdCl2 (2 mg Cd/kg/day, s.c. injection) for 7-14 days. After 7 days of cadmium exposure, urinary excretion of $Na^+$ was markedly reduced. This change was accompanied by an increase in $Na^+$-$K^+$-ATPase activity, a fall in tissue $Na^+$ content, a rise in tissue $K^+$ content and an elevation of plasma aldosterone level.