PURPOSE: This study explored the effect of acute aerobic exercise on cognitive performance in trained cyclists/triathletes. Specifically, it investigated the impact of different exercise durations (completed at a fixed moderate-intensity) and high/maximal intensity exercise (H/MIE) to volitional exhaustion (following a sustained bout of dehydrating activity) on simple and complex cognitive skills. METHODS: On two separate occasions, 21 trained cyclists/triathletes; 11 male (M) (age: 31±8 y; VO2max: 57±9 mL.kg-1.min-1) and 10 female (F) (34±7 y; VO2max: 51±9 mL.kg-1.min-1), completed 45 min of fixed-intensity cycling (M: 80±8%; F: 74±5% HRmax) followed immediately by an incremental test to volitional exhaustion. Cognitive function was assessed at Baseline, after 15 and 45 min of exercise (15EX; 45EX), and at Exhaustion using a 4-choice reaction time (CRT) and Stroop Word-Color Association test (Incongruent/Congruent Reaction Time [RT]). A placebo treatment (“to improve cognition”) was administered after 15EX on one trial to determine if positive expectancy influenced cognitive responses. RESULTS: Exercise Duration: CRT, Congruent RT and Incongruent RT decreased (improved) at 15EX and 45EX compared to Baseline (p's<0.005). While CRT and Congruent RT were faster at 45EX than 15EX (p's<0.020), Incongruent RT was not (p=1.000). Exercise Intensity: The incremental test lasted ˜11.4±2.8 min, with participants achieving a maximum heart rate (HR) equal to ˜93±7% HRmax. CRT, Congruent RT and Incongruent RT decreased at Exhaustion compared to Baseline, (p's<0.005), despite large fluid losses (M: -2.3±0.3% BM; F: -1.7±0.3% BM). The placebo treatment did not affect cognitive responses to H/MIE (p's>0.05). CONCLUSIONS: Acute aerobic exercise improves cognitive performance in trained athletes. These effects are more pronounced when exercising for longer durations (˜1hr), employing higher exercise intensities and/or more complex cognitive tasks.