A recent report1 from this laboratory has demonstrated a greater sodium excretion during fasting than would be expected during a comparable period on a salt-free diet. These observations might suggest that either food in general or some constituent of the diet might be involved in the regulation of sodium excretion. Gamble2 had previously shown that glucose or food diminished the sodium excretion of life raft survivors. That study compared a period of salt-free nutrition with glucose with a period of fasting, using the subject as his own control. The present investigation is concerned with the effects of dietary constituents (sodium chloride, carbohydrate, or protein and fat) on the established pattern of salt excretion of fasting. It has been observed by Gamble2 as well as Hervey and McCance3 that less sodium was excreted in the urine when periods of carbohydrate intake in amounts insufficient for caloric needs