International Journal of Exercise Science 12(6): 1322-1333, 2019. This study examined voluntary fluid intake, hydration descriptors, and sweat loss estimation accuracy following runs in wet bulb globe temperatures of 18 (TEMP) and 26 ºC (HOT). Twelve male runners completed 1-h runs at 65% of VO2 max with access to water during runs and a variety of beverages for the following 24-h. Urine specific gravity (USG), body mass, fluid intake, and urine output were assessed at 12 and 24-h. Runners lost 1.355 ± 0.263 and 1.943 ± 0.485 L during TEMP and HOT, respectively. Sweat loss volume was underestimated by approximately one-third during both conditions. Cumulative fluid intake from start until 1-h post-run was greater in HOT, but not at 12-h (2.202±0.600 vs 2.265±0.673 L) or 24-h (3.602±0.807 vs 3.742±1.205 L). Runners replaced a lower percentage of sweat losses and displayed higher USG (p \u3c 0.001) for HOT (119±34%; 1.027±0.004) versus TEMP (166±51%; 1.018±0.004) at 12-h while exhibiting repeatable rehydration patterns within runners (ICC = 0.89) between trials. Absolute body mass was unable to differentiate the substantial differences in fluid replacement percentage. Seven runners replace
