A method and system for stunning poultry with carbon dioxide gas includes initially establishing an initial elevated CO2 concentration in an initial stun chamber, and then lowering a cage for live poultry downwardly through an open top of the initial stun chamber. The cage is held there for an initial time period, after which the cage is moved into an intermediate stun chamber, where an intermediate elevated CO2 concentration is established. The cage is held in the intermediate stun chamber for an intermediate period of time, after which it is moved into a final stun chamber, where a final elevated CO2 concentration is established. The cage is held in the final stun chamber for a final period of time, and the cage is then raised upwardly through an open top of the final stun chamber and out into an ambient atmosphere.