The sweat of single sweat glands of healthy individuals and cystic fibrosis patients was analyzed for differences in bicarbonate, lactate, and pH. These values were monitored as a function of sweat rate simultaneously with 1) the electrical potential difference at the duct orifice (with respect to the interstitial fluid) and 2) the concentrations of sodium, potassium, and chloride in surface sweat. Sweat in both groups contained about equal concentrations of lactate and bicarbonate at similar sweat rates. Similarly, the pH of sweat secreted at similar rates in the two populations was not significantly different. Acidification of sweat increased with decreasing sweat rate. In both populations, lactate and bicarbonate may be absorbed passively in the distal sweat duct in their nonionic form and, thus, not influenced by the increased electrical potential difference in the cystic fibrosis sweat duct. The uptake of these ions may involve active proton secretion by sweat duct cells.