Operation Everest II: ventilatory adaptation during gradual decompression to extreme altitude. To assess the ventilatory adaptation during gradual ascent to extreme altitude, we studied seven healthy males as part of the 40 d simulated ascent of Mt. Everest in a hypobaric chamber. We measured resting ventilation (VE, l.min-1), arterial oxygen saturation (SaO2%), the ventilatory response to oxygen breathing, isocapnic hypoxic ventilatory response (HVR), and hypercapnic ventilatory response (HCVR) at sea level prior to the ascent (760 torr), 14,000 feet (428 torr), 24,000 feet (305 torr), and within 24 h of descent (765 torr). VE increased from 9.3 +/- 1.1 l.min-1 at 760 torr to 23.4 +/- 1.3 l.min-1 at 305 torr and remained elevated at 14.7 +/- 0.7 l.min-1 after descent. Oxygen breathing decreased VE by 9.6 +/- 1.3 l.min-1 at 305 torr. Isocapnic HVR (expressed as a positive slope of VE/SaO2, l.min-1.%SaO2(-1) increased from 0.18 +/- 0.07 at 760 torr to 0.34 +/- 0.11 and 0.38 +/- 0.5 at 428 torr and 305 torr (P less than 0.05) respectively. HVR was elevated further upon return to sea level (0.8 +/- 0.09, P less than 0.05). HCVR (S = VE/PETCO2, l.min-1.torr-1) increased from sea level (S = 4.4 +/- 0.09) to 305 torr (S = 18.7 +/- 3.5, P less than 0.01) and remained elevated upon return to sea level (S = 10.7 +/- 4.6, P less than 0.001). This study is the first to investigate the ventilatory response to such extreme altitude and so soon after descent and shows that hypoxic and hypercapnic responses increase during prolonged progressive hypoxic exposure and remain significantly elevated from pre-ascent levels immediately upon descent. 