The purpose of this investigation was to examine the combined effects of hydration and exercise heat stress on choice reaction time. On three separate occasions eight male subjects performed cycle exercise at ∼70% of peak power output in warm, humid conditions (31°C, 63% relative humidity) for a maximum of 90min or until exhaustion. Throughout the trials, subjects ingested either a volume of water equal to fluid loss (100−FR), a volume equivalent to ∼50% of fluid loss (50−FR), or no fluid (0−FR). A choice reaction time task was undertaken at rest, after 20min of cycling, 40min of cycling and at the conclusion of exercise. Mean reaction time for 100−FR was 342.2±8.2ms, 352.4±7.5ms for 50−FR and 345.6±8.4ms for 0−FR and were not significantly different. Choice reaction time was facilitated as the duration of exercise progressed with reaction time at 40min and conclusion stages of exercise faster than at rest (P<0.005). Choice reaction time and accuracy were affected by the number of choices, with choice reaction time increasing linearly with the number of choices (P<0.005) and rate of incorrect responses increasing in the 4-choice task compared to the 1-choice and 2-choice task (P<0.05). The results indicate that, in up to 90min of exercise in warm, humid conditions, choice reaction time is not compromised by different levels of hydration.