Fourteen young men (on the average 25 years), well trained (maximal oxygen consumption, VO2 max., between 2.7 to 3.5 1) have been studied at two different levels of exercise: 90 and 140 watts (about 40 and 70% of VO2 max.) in chamber, where the atmosphere was regulated. The subjects performed the exercise after a sojourn of six hours in the chamber, at the same level, either in air or in hypercapnic conditions (FICO2: 0.04; FIO2: 0.21); the order of the exercise tests was determined at random. The rise of total ventilation (VE) during exercise in CO2 atmosphere was particularly related to the increased tidal volume (VT). In spite of the larger increase of VE in hypercapnia, CO2 output (VCO2) and respiratory quotient (R) were lower while PaCO2 was elevated (48 at rest and 54 mmHg during exercise). Oxygen consumption during exercise was the same in both conditions. Values of arterial lactic acid concentration were not different at 90 watts level. On the contrary, at the level of 140 watts, the lactic acid concentration was significantly lower in CO2 atmosphere. The well known changes during exercise of other electrolytes (rise of Na+, K+ and total Ca) was similar in air and in CO2. Only the inorganic phosphorus was higher in CO2 atmosphere at 140 watts.