Effects of 21-d dry immersion (DI) on ventilation response to increasing hypercapnia were studied using the Starr "calm down" breathing. Ventilation response to СО2 (SVСО2) and changes in the respiration capacity due to growing hypercapnia (SVtCO2) were measured before, during and on completion of 21-d DI. Measurements in the course of DI included residual pulmonary volume (RPV), functional residual capacity (FRC) and CO transfer-coefficient of the lung (KCO). SVСО2 and SVtCO2 were determined within the CO2 partial pressure range of 45 - 60 mm Hg in the end-exhaled air (PETCO2). Since the subjects breathed an oxygen gas mixture and the hypoxic response could not develop, SVСО2 was determined by PETCO2 only. On DI days 13 and 19, group-average SVtCO2 was significantly (р< 0.05) higher than the baseline value by 0.023 ± 0.019 and 0.018 ± 0.016 l/mmHg, respectively. On DI day 13, SVtCO2 was significantly (р< 0.05) higher by 0.020 ± 0.023l/mmHg when compared to DI day 8. Throughout the DI study, the CO2-related apneic threshold tended to rise; on DI day 13, the threshold was significantly higher (Р< 0.05) when compared to the value on DI day 8 (by 3.09 ± 4.07 mmHg). There was also a significant increase (Р< 0.05) of the breathing capacity at the apneic threshold on DI day 13 when compared to the baseline value (by 6.47 ± 7.09 mmHg) and on day 8 (by 7.27 ± 9.15 mmHg). TLP decreased significantly (Р< 0.05) on DI day 13 when compared to day 8 (by 0.14 ± 0.13 l).