This study was conducted to determine the reaction of broilers when they first come into contact with various gas mixtures, some of which can be used for stunning prior to slaughter. The birds were exposed to either air, 90% argon, 60% CO2, 40% CO2 and 30% O2, or 70% argon and 30% CO2. There were no differences in behaviour before entering the chamber or when the birds first arrived. Headshaking and gasping are typical reactions to CO2. The number of birds showing such behaviour was least in the 90% argon mixture, and was reduced in the CO2/O2 mixture compared to the other combinations that included CO2. Loss of posture (falling) is indicative of the animal loosing conciousness. Among the non-air treatments, this occurred first in the Argon/CO2 combination. In conclusion, the birds did not appear to detect or avoid increased CO2 levels when first exposed to the mixtures, high levels of O2 in the mixtures reduced the negative reactions of birds exposed to CO2, and Argon was effective in achieving rapid loss of consciousness.