The aim of this study was to examine the effect of a running bout under hot conditions on cognitive performance in physically active men. Sixteen participants ran at 60% of maximum aerobic speed for an average time of 52.4 +/- 7.6 minutes under hot environmental conditions (35 degrees C, 60% relative humidity). Changes in body mass, lean mass, hematocrit, plasma volume, serum urea, creatinine, and thirst score were assessed to evaluate the state of hydration immediately before and after exercise. Cognitive performance was assessed using the Vienna Test System battery before and after exercise. The running protocol led to a decreased body mass, lean mass, plasma volume and an increased hematocrit, serum urea, creatinine and thirst score (all p < 0.05), implying that there was significant impairment in the state of hydration. After the running bout, complex and peripheral reaction time consistently improved, whereas visual angle was impaired (all p < 0.05). A running bout in the heat improves the speed of response in complex tasks but impairs the field of vision and leads to a deleterious hydration stare. [J Exerc Sci Fit . Vol 9 . No 1 . 58-64 . 2011]