In 78 healthy subjects, at 760 m (Frunze) and 3600 m (the Pamirs) above sea level, apneic points and ventilatory responses to increasing hypercapnia were assessed (rebreathing method). At 3600 m (4 to 11-month adaptation), regulation of breathing was achieved at lesser threshold CO2 values than in lowlanders, with no quantitative difference in ventilatory responses. On days 5 to 7 of deadaptation (at 760 m) following 45 days of adaptation at 3870 m, there was a decrease in ventilatory responses to a 1 mm Hg increment in PACO2, while apneic point values approached base-line values (prior to ascent to the high altitude).