Davis, JK, Laurent, CM, Allen, KE, Zhang, Y, Stolworthy, NI, Welch, TR, and Nevett, ME. Influence of clothing on thermoregulation and comfort during exercise in the heat. J Strength Cond Res 31(12): 3435-3443, 2017-Sport textiles of synthetic fiber have been proposed to have superior properties for keeping wearers cooler, drier, and more comfortable compared with natural fibers. The impact of various fiber content and fabric construction on thermoregulation and perceptual responses are not well understood. Eight male collegiate athletes performed 3 counterbalanced trials of 45-minute treadmill run at 60% of maximal oxygen uptake in an environmental chamber (32° C). Three different fibers, consisting of 100% cotton, a blend of natural fibers (50/50% cotton/soybean), and a synthetic fiber (100% polyester) with mesh loops to facilitate ventilation through the clothing, were tested. Heat strain indices, microenvironment temperature, ratings of perceived exertion (RPE), and clothing comfort were measured. Session RPE (S-RPE) and session thermal sensation (S-TS) were recorded 20 minutes after each trial. There was no effect of clothing on rectal, skin, and body temperatures, heart rate, RPE, or comfort measures (p ≥ 0.05). A significant effect was observed for synthetic fiber compared with cotton on S-RPE (p = 0.03), S-TS (p = 0.04), and the microenvironment temperature at the chest (p = 0.02). No significant difference was shown for any other fibers on S-RPE, S-TS, or other microenvironment areas (p ≥ 0.05). These results show that clothing fiber content and fabric construction had no effect on thermoregulation, RPE, or clothing comfort during moderate-intensity exercise in the heat; whereas synthetic fabric construction indeed effectively reduced regional microenvironment temperature and attenuated global exertion and TS, which may have important implications for exercise tolerance in the heat.