Following an overnight fast and oral hydration, some normal individuals developed a marked natriuresis. This renal salt wasting could be promptly curtailed by the ingestion, or in some cases, infusion of glucose. Infusion of insulin alone also was capable of decreasing urinary sodium excretion. Older subjects appeared more prone to develop an exaggerated natriuresis following fasting and hydration and developed more striking salt retention following carbohydrate ingestion than did younger subjects. Young subjects, restudied after longer periods of fasting (36 hours), still failed to develop a natriuresis following hydration. The hypothesis that the hyponatremia in acutely and chronically ill patients might develop, at least in part, as a result of excess salt loss during periods of inadequate carbohydrate intake was tested. Acutely and chronically ill patients, many with mild hyponatremia, failed to develop inappropriate salt loss with overnight fast and hydration. These findings failed to support this hypothesis.