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The increasing use of prolonged Na restriction in the treatment of cardiovascular diseases in particular, makes study of long-term Na balance pertinent. With diets which contain 2 to 7 mEq. Na per day, combined urine and stool losses average 1 to 5 mEq. Na per day (1, 2). Therefore, over an extended period the loss of electrolyte through the skin might assume importance. Only two earlier studies were found relative to electrolyte losses of the entire skin, both after shortterm Na restriction, namely 3 (3) and 14 (4) days.The data from Benedict's classical study (5) of an individual who fasted for one month have relevance to any work on the physiological response to dietary restriction, but the metabolic effects of starvation are not directly applicable to those effected by limitation of only two dietary components-namely sodium and chloride.For almost seven years the authors of the present paper have been using a diet which contains about 6 mEq. Na per day in the study and treatment of patients with essential hypertension. On such a regimen combined urine and stool losses average 2 to 5 mEq. Na per day, and since no evidence of hyponatremia had been found in the absence of serious renal disease, it was reasonable to suppose that the patients were in Na balance. The current study was undertaken to quantitate the actual skin losses of several electrolytes on seven subjects who had been on the low Na regimen for one to five months, during two collection periods of 7 and 3 days. Although interest was centered primarily on Na, Cl and K were measured as well. MATERIALThe seven white adult subjects had been admitted to the metabolic wards of the Brookhaven National Laboratory Hospital for studies relative to the hypertension 1 This research was sponsored by the Atomic EnergyCommission.research program. The pertinent clinical and laboratory data on these patients are summarized in Table I. It will be seen that five of the patients, all females, had classical uncomplicated benign hypertension. In two (Di, Jo), the blood pressure had attained near-normal levels by the end of the six-weec control period and be-