This study examined the effect of prolonged exercise with and without hyperthermia on cognitive function. In this study, 9 male subjects cycled at 50% watt max for 60 min in a room maintained at 23°C. Water-perfused suits with water at 7°C (COOL) and 47°C (HOT) were randomly used during the 2 cycling periods. Rectal temperature, skin temperatures, heart rate, ratings of perceived exertion (RPE), and thermal sensation were evaluated during exercise. The Stroop color-word test (SCWT) and executive processing test (EPT) were conducted every 15min during exercise to evaluate cognitive function. In the COOL trial, the core temperature stabilized at 37.5 ± 0.1°C (mean ± SE), whereas in the HOT trial, the core temperature increased to 39.1 ± 0.1°C, with a concomitant increase in RPE and thermal sensation. Although the reaction time for the simple task of the SCWT improved when the body temperature increased slightly during the COOL trial, the conflict task of the SCWT was impaired during the latter stages of exercise during the HOT trial. The reaction time for the HOT trial was significantly longer than that for the COOL trial. Compared to the bigining of exercise the reaction time for the EPT was impaired at 60 min during the HOT trial, and this reaction time was longer than that for the COOL trial. The present findings indicate that exercise-induced hyperthermia during the latter stages of exercise impairs cognitive as well as physiological functions.