In normal humans, when sodium intake is abruptly reduced from a high to a very low level, renal sodium excretion falls exponentially (half time approximately 24 h), and several days are required to achieve external sodium balance, where intake equals excretion. Because much of our knowledge of intrarenal mechanisms comes from the rat, we studied their capacity to handle sodium. In two strains of rat, Sprague-Dawley (SD) and Wistar-Kyoto (WKY), whether the sodium load was administered intravenously, by gavage, or by spontaneous feeding, the slope relating sodium excretion to time was 8-10 times more rapid than in humans, reflecting half times of 2-3 h, and external sodium balance was achieved in hours rather than days. The combination of normal rat nocturnal feeding patterns and the rapidity of the response result in a daily spontaneous transition from an expanded state with a high or intermediate level of sodium excretion to a more contracted state, with minimal sodium excretion. Studies designed to assess sodium homeostasis in rats, and related renal and hormonal changes, must consider these rapid transitions, which are related, perhaps, to the rats' persistent and remarkably rapid growth.