Exposure to high altitude induces physiological or pathological modifications that are not always clearly attributable to a specific environmental factor: hypoxia, cold, stress, inadequate food. The principal goal of hypobaric chamber studies is to determine the specific effect of hypoxia. Eight male volunteers ("altinauts"), aged 23 to 37 were selected. They were first preacclimatized in the Observatoire Vallot (4,350m) before entering the chamber. The chamber was progressively decompressed down to 253 mmHg barometric pressure, with a recovery period of 3 days at 5,000m in the middle of the decompression period. They spent a total of 31 days in the chamber. Eighteen protocols were organized by 14 European teams, exploring the limiting factors of physical and psychological performance, and the pathophysiology of acute mountain sickness (AMS). Ali subjects reached 8,000m and 7 of them reached the simulated altitude of 8,848m. Three altinauts complained of transient neurological symptoms which resolved rapidly with reoxygenation. Body weight decreased by 5.4 kg through a negative calorie balance. Only four days after the retum to sea-level, subjects had recovered 3.4 kg, i.e. 63% of the total loss. At 8,848m (n=5), PaO2 was 30.6 ± 1.4 mmHg, PCO2 11.9 ± 1.4 mmHg, pH 7.58 ± 0.02 (arterialized capillary blood). Hemoglobin conce_ntration increased from 14.8±1.4 to 18.4±1.5 g/dl at 8,000m and recovered within 4 days at sea-level. AMS score increased rapidly at 6,000m and was maximal at 7 ,000m, especially for sleep. AM~ was related to alteration in color vision and elevation of body temperature. V02w.x decreased by 59% at 7 ,O00m. The purpose of this paper is to give a general description of the study and the time course of the main clinicat and physiological parameters. The altinauts reached the "summit" (for some of them three consecutive times) in better physiological conditions than it would have been possible in the mountains, probably because acclimatization and other environmental factors such as cold and nutrition were controlled.