The effect of water deprivation for 1 day on Na balance was measured in rats, rabbits, and sheep. In all three species, dehydration induced a considerable increase in the daily output of Na in urine. This resulted in the water-deprived animals becoming Na depleted as well as water deficient. Experiments were also designed to control for the reduced food intake that results from water deprivation in rats and rabbits. The results showed that decreased food intake was not the cause of the natriuresis and subsequent Na depletion that occurred during dehydration. When water was again made available to the dehydrated animals, a period of urinary Na retention ensued. This Na retention usually continued until the Na deficit that had accrued during the period of dehydration had been ameliorated. It seems likely that natriuresis in response to dehydration is common in mammals. We postulate that such a natriuretic response may serve a homeostatic function in buffering increases in plasma [Na] and osmolality that result from dehydration.