Changes in sodium and potassium balance and urinary aldosterone excretion (free plus “acid labile”) were surveyed in house mice acclimating initially to 1/2 the ad libitum water ration and later to 1/4 of ad libitum. Each water restriction caused negative sodium and potassium balance, followed by conservation of both ions. At the first water restriction, negative balance was produced by natriuresis and kaliuresis, at successive water restriction, a negative balance was due to a decrease in food consumption. Aldosterone excretion doubled at the 1/2 ad libitum restriction, and then generally remained at control levels thereafter. Fractional intestinal absorption of both ions showed a persistent increase after the 3rd day of water restriction. The Na:K ratio in urine and feces declined as mice responded to each water restriction and returned to control levels in acclimated animals.