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The present investigation aimed to study the cardiovascular responses and the cerebral oxygenation (Cox) during exercise in acute hypoxia (AH) and with contemporary mental stress. Fifteen physically active, healthy males (age 29.0 ± 5.9 years) completed a cardiopulmonary test on a cycle ergometer to determine the workload at their gas exchange threshold (GET). On a separate day, participants performed two randomly assigned exercise tests pedalling for six minutes at a workload corresponding to 80% of the GET: 1) during normoxia (NORMO), and 2) during acute, normobaric hypoxia at 13.5% inspired oxygen (HYPO). During the last three minutes of the exercise, they also performed a mental task (MT). Hemodynamics were assessed with impedance cardiography, and peripheral arterial oxygen saturation and Cox were continuously measured by near infrared spectroscopy. The main results were that both in NORMO and HYPO conditions, the MT caused a significant increase in the heart rate and ventricular filling rate. Moreover, MT significantly reduced (74.8 ± 5.5 vs. 62.0 ± 5.2 A.U.) COX while the Reaction Time (RT) increased (813.3 ± 110.2 vs. 868.2 ± 118.1 ms ) during the HYPO test without affecting the correctness of the answers. We conclude that in young, healthy males, adding a mental task during mild intensity exercise in both normoxia and acute moderate (normobaric) hypoxia induces a similar hemodynamic response. However, mental task and exercise in HYPO causes a decrease in COX and an impairment in RT.