MANGOS, JOHN. Transductal fluxes of Na, K, and water in the human eccrine sweat gland. Am. J. Physiol. 224 (5): 1235-1240. 1973.--The bidirectional fluxes of Na, the net influx of K, and the net fluxes of water were studied in the human sweat duct in vitro by free-flow microperfusion with sweat resembling the primary secretory fluid and by microanalysis of the fluid samples obtained. Full-thickness skin explants were mounted in a specially designed chamber and single sweat ducts were perfused. The transductal water fluxes were small. At perfusion rates of 17.3, 12.7, and 5.2 nl/duct min, the net Na reabsorption was 0.418, 0.504, and 0.584 nEq/rnin, whereas the calculated Na backdiffusion was 0.208, 0.190, and 0.049 nEq/min, respectively. These, opposing changes resulted in constant values of total Na reabsorption. Net K influx was a small fraction of Na reabsorption with values of 0.047, 0.070, and 0.043 nEq/min at the three perfusion rates. Addition of ouabain, 5 X 10-SM, to the bath medium decreased both the Na reabsorption and K influx.