Cross-sectional studies in endurance athletes have demonstrated a diminished hypoxic ventilatory response (HVR) compared with mountaineers or sedentary controls. Conversely, short-term altitude acclimatization may increase the HVR. The longitudinal effect of training, either at sea level or altitude, on HVR has not been previously reported. We therefore studied 21 untrained men and women before and after 5 wk of cycle ergometer training at either sea level or 2,500 m. HVR was determined using the steady-state method (16). Minute ventilation (VE) was measured with a Tissot spirometer during the last minute of 5 min breathing room air, 8% and 12% O2, administered in random order. CO2 was added at the mouth in an effort to maintain end-tidal CO2 at baseline levels. Oxyhemoglobin saturation was measured directly from arterial blood with a hemoximeter (OSM 3). HVR was defined as the positive slope of the line relating VE to O2 saturation in l.min-1%-1. One group of subjects trained at sea level at 70% maximal oxygen uptake (VO2max; N = 7). A second group trained at 2,500 m in a hypobaric chamber, at the same relative exercise intensity (i.e., 70% altitude VO2max) or same absolute intensity (same power output) as group 1 (N = 14). Both groups trained on a bicycle ergometer for 45 min.d-1, 5 d.wk-1 for 5 wk.(ABSTRACT TRUNCATED AT 250 WORDS)