Measurements in alveolar gas have suggested extreme hypocapnia and alkalosis on the summit of Mt. Everest. However, tensions in both expired gas and arterial blood have not been reported for the summit of Mt. Everest (PIO2 = 43 mm Hg). To approach the problem, we examined alveolar (and end-tidal) and arterial gas tensions in 6 healthy men who completed a 40-d chamber study to the simulated "summit," with 20 d above 6,400 m and 9 d above 8,000 m. In 27 simultaneous samples of alveolar air and arterial blood for inspired oxygen tensions ranging from PIO2 of 55 mm Hg (7,380 m) to 43 mm Hg, the mean alveolar-arterial difference was negligible for PO2 (-0.8 +/- 2.4 (S.D.) mm Hg) and PCO2 (0.5 +/- 1.4 mm Hg). For all 6 subjects at the summit, PACO2 was 12.0 +/- 1.8 and PACO2 was 11.4 +/- 1.6 mm Hg, and for the two with the lowest values, alveolar and arterial PCO2, respectively, were 9.5 and 9.8 mm Hg. Arterial pH averaged 7.53 units. We conclude that while acclimatization to severe hypoxia results in extreme hypocapnia, alkalosis is more moderate than previously reported. Alveolar gas tensions reflect well the values obtained in arterial blood.