Carlson, LA, Lawrence, MA, and Kenefick, RW. Hydration status and thermoregulatory responses in drivers during competitive racing. J Strength Cond Res 32(7): 2061-2065, 2018-Stock car drivers are exposed to high ambient temperatures, further complicated by the fact that they are encapsulated in protective clothing; however, the hydration status of these drivers has not been determined. This study quantified the degree of fluid losses during a competitive event in hot conditions. Nine male stock car drivers (29.6 ± 9.4 years, 177.8 ± 3.0 cm, 81.5 ± 18.5 kg) were studied during a Pro Series Division NASCAR race. Sweat rate (SR) and dehydration was determined through nude body weights (BWs). Prerace BW was 81.5 ± 18.5 kg and decreased to 81.1 ± 18.5 kg after race (p = 0.001). Body weight loss after race was 0.77 ± 0.3% and mean SR was 0.63 ± 0.4 L·h. Intestinal core temperature increased from 38.0 ± 0.4 to 38.5 ± 0.4° C after race (p = 0.001). Skin temperature increased from 35.8 ± 0.8 to 36.9 ± 0.8° C after race (p = 0.001), whereas the core-to-skin temperature gradient narrowed from 2.2 ± 0.9 to 1.6 ± 0.9° C, before race to after race (p = 0.001). Heart rates after race were 89 ± 0.0% of the drivers' age-predicted maximum heart rate (HR). Fluid losses during competitive racing can be significant. Without a fluid replacement strategy, fluid losses may exceed 3% of BW and could negatively impact driving performance in longer races.