The natriuresis of fasting has been well characterized in man and rabbits but not in rats. The daily effects of fasting on glomerular filtration rate (GFR) and urinary sodium and potassium excretion were evaluated in Munich-Wistar rats (260-310 g) submitted to prolonged starvation (2-8 days). Rats do not present the natriuresis of fasting. Sodium excretion was reduced since the first few hours (0-4 h) of starvation. Antinatriuresis was abrupt during the early periods (1st and 2nd days) and stabilized at very low levels. During the early phase (4 days), sodium retention occurred due to both reduced glomerular filtration and increased tubular reabsorption. However, during the late phase (after the 4th day), antinatriuresis was mainly induced by the elevation in tubular reabsorption, since a normalization of GFR was observed. Thus, these homeostatic mechanisms permit adequate renal sodium conservation during starvation in rats.