Isocaloric 5.61 mJ (1,340 kcal) formula diets involving the isocaloric exchange of fat and carbohydrate were fed to 21 obese persons selected for sex, height, and weight before the start of the treatment and distributed over three groups. The weight loss observed during the carbohydrate-restricted diets was significantly greater than during the high-carbohydrate diet. After 28 days of treatment the weight loss recorded on the high-carbohydrate diet was 9.5 +/- 0.7 kg, as compared to 11.4 +/- 0.7 kg (p less than 0.05) on the corn oil-containing diet and 12.5 +/- 0.9 kg (p less than 0.01) on the butter-fat-containing diet. The weight loss achieved was not dependent on the type of fat administered (saturated vs. polyunsaturated). When calculated cumulatively, sodium excretion during the first 7 days was significantly greater on the low-carbohydrate diet, whereas after 28 days the total amount of sodium excreted was highest on the high-carbohydrate diet. Potassium excretion during the low-carbohydrate diets was significantly greater for as long as 14 days, but at the end of the experimental period the observed differences no longer attained statistical significance. At no time did the intake and loss of fluid and the balances calculated therefrom show significant differences. From the findings obtained it appears that the alterations in the water and electrolyte balance observed during the low-carbohydrate diets are reversible phenomena and should thus not be regarded as causal agents of the different weight reduction.