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Eccrine sweat in humans is normally a hypotonic solution, the solutes of which consist mainly of sodium, chloride, potassium, lactate, and urea (1). The sodium and chloride concentrations increase with increasing sweat rate (2), and at moderate to high sweat rates, the sodium concentration is an adequate measure of tonicity (3).It is generally presumed that sweat is elaborated by the secretory coil of the gland and altered during its passage down the duct (4). However, analysis of the physiology of the duct depends upon relatively precise knowledge of the nature of the fluid elaborated at the secretory coil (secretory fluid). Previous studies give values for the sodium concentration of the secretory fluid that are distinctly hypotonic (2, 5) or suggest that it is isotonic (6). Our studies in which we analyzed the relationship of rate of sodium excretion to sweat rate from the forehead give approximately isotonic values for the sodium concentration of the secretory fluid. MethodsSix healthy men, ages 21 to 29, were used as subjects. For 1 week before the experiment, the only exercise allowed was that necessary for the usual duties of a dermatology resident or laboratory assistant. The The skin of the forehead was gently cleansed with soap and water, rinsed with ion-free distilled water, and air dried. Aluminum chambers with internal diameter of 3.8 cm as described by Schwartz and Thaysen (2) were secured to the skin with Weldwood contact cement. Two Whatman 540 filter papers, 3.7 cm in diameter, were removed from weighed plastic bottles and introduced into the chamber and sealed at time zero. The filter papers were collected and replaced in their weighing bottles, and fresh preweighed filter papers were inserted every 3 to 10 minutes depending upon an estimate of the volume of sweat being produced. The time required for changing the filter papers was usually less than 5 seconds. Eight to twelve collections were obtained from each subject.The subjects, wearing swimming trunks, entered a room with temperatures ranging from about 950 to 1100 F and humidity around 40%. By altering the temperature and humidity in conjunction with mild, moderate, and vigorous exercise in the form of calisthenics, varying sweat rates were produced. At some time during the experiments the subjects were encouraged to maximal tolerated exercise. Sweat rates were increased and decreased so as to preclude sweat rate being a function of time. The subjects were weighed before and after the experiment, but were allowed water ad libitum during the experiment. The maximal duration of the experiment was 75 minutes for one subject and 60 minutes for the other five.Sweat volumes were determined by weight differences of the filter papers. The filter papers were then eluted with ion-free distilled water and the sodium concentrations determined on appropriate dilutions with a Perkin Elmer atomic absorption spectrophotometer. Suitable blanks were used for each subject. Recovery experiments in which 0.1 ml of a 100 mEq per L sodium chloride solution wa...

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