This study examined the effects of increasing the percentage of carbon dioxide (CO2) in the atmosphere of group-housed broilers as a method of euthanasia. Measuring the level of discomfort before loss of consciousness, and measuring the killing rate, assessed the suitability of the methods used for 2- and 6-week old broilers. The gas mixtures used in the trial for injection into the air were 1) 100 % CO2, 2) 50 % N2 + 50 % CO2, and 3) 30 % O2 + 40 % CO2 + 30 % N2, followed by 100 % CO2. At the time of injection of the gas mixture into the housing, 2 birds per trial that were equipped with brain electrodes were observed for behavior and brain activity. Of the 2-week old birds, 10 % survived the increasing CO2 percentage due to the use of mixture 3. Therefore, that particular mixture was not used in further tests on 6-week old birds. Thirty percent of the 6-week old birds survived the 50-50 mixture of CO2 and N2. Using the 100 % CO2 injection into the air, the highest percentage (42 %) of CO2 in the breathing air was reached. Only 25 % of CO2 in the breathing air was reached using the other two methods. When using each of the three mixtures, head shaking, gasping, and convulsions were observed before loss of posture. Suppression of electrical activity in the brain occurred almost simultaneously with loss of posture. The results of this study indicated that euthanasia of groups of 2- and 6-week old broilers by gradually increasing the percentage of CO2 in the breathing air up to 40 % was possible.