The combined effects of hyperventilation and arterial desaturation on cerebral oxygenation (ScO2) were determined using near-infrared spectroscopy. Eleven competitive oarsmen were evaluated during a 6-min maximal ergometer row. The study was randomized in a double-blind fashion with an inspired O2 fraction of 0.21 or 0.30 in a crossover design. During exercise with an inspired O2 fraction of 0.21, the arterial CO2 pressure (35 +/- 1 mmHg; mean +/- SE) and O2 pressure (77 +/- 2 mmHg) as well as the hemoglobin saturation (91.9 +/- 0.7%) were reduced (P < 0.05). ScO2 was reduced from 80 +/- 2 to 63 +/- 2% (P < 0.05), and the near-infrared spectroscopy-determined concentration changes in deoxy- (DeltaHb) and oxyhemoglobin (DeltaHbO2) of the vastus lateralis muscle increased 22 +/- 3 microM and decreased 14 +/- 3 microM, respectively (P < 0.05). Increasing the inspired O2 fraction to 0.30 did not affect ventilation (174 +/- 4 l/min), but arterial CO2 pressure (37 +/- 2 mmHg), O2 pressure (165 +/- 5 mmHg), and hemoglobin O2 saturation (99 +/- 0.1%) increased (P < 0. 05). ScO2 remained close to the resting level during exercise (79 +/- 2 vs. 81 +/- 2%), and although the muscle DeltaHb (18 +/- 2 microM) and DeltaHbO2 (-12 +/- 3 microM) were similar to those established without O2 supplementation, work capacity increased from 389 +/- 11 to 413 +/- 10 W (P < 0.05). These results indicate that an elevated inspiratory O2 fraction increases exercise performance related to maintained cerebral oxygenation rather than to an effect on the working muscles.