Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man. Med. Sci. Sports Exerc., Vol. 29, No. 9, pp. 1240-1249, 1997. Eight healthy males performed four rides to exhaustion at approximately 70% of their ˙VO2max obtained in a neutral environment. Subjects cycled at ambient temperatures (Ta) of 3.6± 0.3 (SD), 10.5 ± 0.5, 20.6 ± 0.2, and 30.5 ± 0.2°C with a relative humidity of 70 ± 2% and an air velocity of approximately 0.7 m·s-1. Weighted mean skin temperature([horizontal bar over]Tsk), rectal temperature (Tre), and heart rate (HR) were recorded at rest, during exercise and at exhaustion. Venous samples were drawn before and during exercise and at exhaustion for determination of hemoglobin, hematocrit, blood metabolites, and serum electrolytes and osmolality. Expired air was collected for calculation of˙VO2 and R which were used to estimate rates of fuel oxidation. Ratings of perceived exertion (RPE) were also obtained. Time to exhaustion was significantly influenced by Ta (P = 0.001): exercise duration was shortest at 30.5°C (51.6 ± 3.7 min) and longest at 10.5°C(93.5 ± 6.2 min). Significant effects of Ta were also observed on ˙VE, ˙VO2, R, estimated fuel oxidation, HR, Tre, [horizontal bar over]Tsk, sweat rate, and RPE. This study demonstrates that there is a clear effect of temperature on exercise capacity which appears to follow an inverted U relationship.