SINCE the discovery of the sweat electrolyte abnormality in cystic fibrosis of the pancreas the attention of investigators has been directed mainly toward concentrations of electrolytes in sweat. Despite the growing interest in the physiological mechanisms involved in determining the final composition of sweat, there has been little emphasis on nonelectrolyte penetration into the sweat gland apart from Schwartz's observation1 that sweat to plasma urea concentration ratios (which had been shown to be greater than 1) do not appear to change with elevations of the plasma urea concentration. (Throughout the remainder of this article the sweat to plasma concentration ratio will be expressed as s/p urea, s/p thiourea, etc.) Schwartz's study led him to conclude that the elevated s/p urea was a passive process dependent on the back diffusion of water.
In the sweat gland at least two mechanisms may be responsible for this postulated back diffusion of water.