The purpose of this study was to measure the in vivo Na⁺ secretion and Na⁺ reabsorption rates of the human eccrine sweat gland with increases in sweat rate. Such data should help to elucidate the physiological mechanism responsible for the previously reported linear relationship between increases in sweat rate and [Na⁺]_sweat. On five days, each subject (n = 10) completed a 30-min exercise bout in an environmental chamber set at 35 °C and 40% relative humidity. The intensity for the 5 exercise bouts in the heat was set to approximate 50, 60, 70, 80 and 90% of age-predicted maximum heart rate. Forearm sweat samples and capillary blood samples were collected during each of the five 30-min exercise bouts. The sweat and blood samples were analyzed for [Na⁺]_sweat and [Na⁺]_serum which were used to calculate the rate of Na⁺secretion and Na⁺ reabsorption. The mean correlation between sweat rate and [Na⁺]_sweat was found to be r = 0.73. Within the sweat rate range of the current study both Na⁺ secretion rate and Na⁺ reabsorption rate increased linearly, however the Na⁺ secretion rate increased almost twice as fast (slope = 141 vs. 80). Thus, the rate at which Na⁺ escaped reabsorption increased with increases in sweat rate and was significantly (P < 0.05) correlated to the [Na⁺]_sweat (mean r = 0.90). Such results strongly suggest that the physiological mechanism responsible for the previously reported linear increase in [Na⁺]_sweat seen with increases in sweat rate is that the Na⁺ secretion rate increases proportionally more than the Na⁺ reabsorption rate.