Changes in weight and body content of protein, carbohydrate, fat, total body water, intra- and extracellular water, Na, and K were estimated from balance measurements in malnourished adult patients receiving total parenteral nutrition for consecutive 8-day periods, containing either a low or high carbohydrate content. Total caloric intake was 65% of energy expenditure on the low intake and 119% on the high intake, with a difference of 54% due entirely to carbohydrate. Although there are many assumptions and approximations necessarily involved in multiple balance studies, the accuracy is sufficient to produce useful information in short-term studies of this type. It is particularly useful in looking at differences between diets, since most of the methodologic errors cancel out. The effect of the increased glucose intake was to significantly increase storage, or decrease loss, of fat, carbohydrate, protein, intracellular water, and K. Changes in Na and extracellular water varied widely among patients, but were not affected by the increase in glucose. Most of the increase in K content (83%) was associated with glycogen deposition; a much smaller amount (17%) was associated with protein deposition. Thus, the ratio of K/N deposited (15 mEq of K:1 g of N) was very different from the 3:1 ratio seen in most tissues. Changes in any one constituent of body cell mass may not, therefore, be good indicators of changes in another. Since protein is the most important active component of body cell mass, changes in body cell mass components other than N should be interpreted with caution when used as indices of nutritional therapy.