In recent studies we have shown that profuse thermogenic sweating alters the structure of the human eccrine sweat gland (1, 2, 3, 4). The amount of sweat secreted, however, does not exert the major influence in determining these structural changes. In a study on the effect of salt intake on sweating, for example, subjects on a low salt intake exhibit far more marked cytologic changes in the eccrine gland than subjects on a high salt intake (4). This occurs even though the rate of sweating was identical in both groups. The excretion of sodium chloride by the sweat gland was greatly influenced by salt intake and this appears to be correlated closely with the structural changes (5). In other words, the lower the rate of excretion of sodium, the greater the cellular change in both secretory coil and duct. These cellular changes can also be correlated with the amount of energy expended by the sweat gland since the production of a dilute sweat requires considerably more metabolic activity than the formation of sweat with a sodium concentration approaching that of plasma. The major unanswered question introduced by these studies, however, is this: Which is the more important factor influencing the structure and function of the sweat gland-is it salt intake, per se, or is it the humoral effects (specifically adrenal cortical) of altered salt intake? To answer this question it was thought appropriate to study the structural changes induced by sweating in subjects on a high salt intake given, in addition, either methyclothiazide (a diuretic) or 9a-fluorohydrocortisone. Methyclothiazide increases the rate of sodium excretion by the sweat gland (probably by blocking the reabsorption of sodium in the duct). 9a-fluorohydrocorti-sone, on the other hand, decreases the rate of sodium excretion by increasing the rate of sodium absorption in the duct (6). With 9a-fluorohydro-cortisone, therefore, the functional effects of a low salt intake could be duplicated, even though subjects were being given an extremely high salt intake. By means of these studies it seemed possible to determine whether salt intake, per se, or the functional effects of salt intake, was the factor most important in determining the structural changes which occur in the eccrine gland with profuse sweating.