Intermittent hypoxic training (IHT) shows promise for prevention and treatment of some diseases and efficiently produces great advancement in athletic training. We studied (1) hypoxic ventilatory responses (HVR) in supine and sitting positions during normobaric, isocapnic, progressive hypoxia (rebreathing technique) and (2) lung ventilation and gas exchange while breathing ambient air at rest and during 5 min of breathing 11% O2. Duel measurements were made pre- and post-15-day IHT regimen on 12 (experimental) healthy males (24.6 y.o. +/- 1.9 y.o.) and on 6 (control) healthy males (24.2 y.o. +/- 2.3 y.o.) given pseudo-IHT (p-IHT) without decreasing PiO2. IHT involved rebreathing eucapnic (chemically absorbed) air as P(ET)O2 decreased to 35 mmHg, three 6-7 min sessions, three times a day, with 10 min breaks between each session over a 15 day training period. Without IHT, HVRs were the same in sitting and supine positions at low levels of hypoxic challenge (slope one--S1: P(ET)O2 from 110-60 mm Hg) and significantly higher (by 45%) during severe hypoxia (slope two--S2: P(ET)O2 from 60-35 mm Hg). IHT caused an increase in HVR in both sitting and supine positions: S1 by 70 and 100%, S2 by 158 and 200%, maximal lung ventilation by 35 and 78%, respectively. There were no significant changes in the p-IHT group. IHT also caused enhanced respiratory reactions during sustained hypoxia (lung and alveolar ventilation increased by 36 and 22%, respectively). A striking hypoxic ventilatory sensitivity was noted in subjects with hyper-reactive breathing patterns.