The effects of chronic exposure to high altitudes on physiology have been extensively investigated and are well understood (1). However, less research has focussed on the affect of altitude on psychological and cognitive performance and studies that have report conflicting findings (2, 3). The purpose of this study was to determine the effect of acute normobaric hypoxia in isolation and combined with light sub-maximal exercise on cognitive performance. Twelve volunteers (8 women; age 20±1 years; height 175±10 cm; body mass 69±10kg) completed the study. Following familiarisation and practice sessions participants completed experimental sessions in normoxia (ambient temperature 19°C; relative humidity 50%) and hypoxia (5500 m equivalent altitude; O2 10.5%; CO2 0.04%; N2 balance; ambient temperature 19°C; relative humidity 50%). Sessions were completed following a crossover design and separate by 10 days. Each session comprised a 20 min stepping exercise divided into four 5 min bouts (4 min stepping at 50 steps/min, 1 min seated rest) preceded and followed by a Stroop test. Arterial oxygen saturation (SpO2), heart rate and ratings of perceived exertion (RPE) were recorded before and after each exercise session. Data were analysed with paired t-tests or a Wilcoxon signed-rank tests, as appropriate, p<0.05 was regarded as statistically significant. There were no differences between conditions for heart rate or RPE pre-exercise, but following the last bout both were greater in hypoxia (normoxia vs. hypoxia: HR: 95 vs. 126 beats/min; RPE: 8 vs. 13; p<0.01). In hypoxia SpO2 was lower both pre- and post-exercise (97 vs. 74%, p<0.01). There was no effect of exercise or hypoxia on Stroop test errors. There were marginal changes in RT after exercise with normoxia decreasing by 3% and hypoxia increasing by 7%, but neither significantly. Results suggest that complex reaction time is not affected by an acute exposure to hypoxia in isolation or when combined with light exercise.