• The intravenous injection of 250 μc of sodium radiosulfate containing S35 yields a figure called the radiosulfate space, an estimate of the extracellular water of the body. This figure, divided by the patient's weight, gave 16.1% as the average extracellular water in 30 normal subjects. The average for patients with congestive heart failure without palpable edema was 22.5%. Simultaneous injection of erythrocytes labeled with 15 pc of radiochromium permitted determination of the blood volume in the same subjects. No significant difference in blood volume was found between the normals and the patients with congestive heart failure without edema. Calculations based on these facts showed that a patient in mild heart failure might have an increase of 5 liters in extracellular water without palpable edema. Therefore the determination of the radiosulfate space should help in the diagnosis of latent or impending cardiac edema. Uncomplicated hypertension in 11 subjects was not accompanied by demonstrable changes in either blood volume or radiosulfate space.