Facility for stunning poultry comprising a poultry treatment tunnel, said tunnel comprising at least: - a first chamber equipped with an opening for introducing the poultry into said tunnel and with first conveying means for conveying the poultry through the first chamber to an opening for transferring the poultry, - a second chamber equipped with an opening for receiving the poultry and with second conveying means for conveying the poultry through the second chamber to an opening for evacuating the poultry from said tunnel, - an intermediate pass-through chamber separating the first and second chambers (100, 102) and communicating, at the ends thereof, with the transfer opening of the first chamber and the receiving opening of the second chamber, said intermediate chamber being equipped with third conveying means for conveying the poultry through the intermediate chamber, the first and second chambers (100, 102) being provided with stunning gas supply means ensuring an increase in the concentration of stunning gas from the opening for introducing the poultry into the tunnel to the opening for evacuating the poultry from said tunnel, said tunnel comprising: - a lower portion on which said first, second and third conveying means (100A, 101A, 102A) are mounted, and - an upper portion comprising an upper wall connecting side walls, said upper portion resting on the lower portion and being connected to lifting means intended to raise said upper portion relative to said lower portion, said facility being characterised in that it comprises at least one port (130, 131) for injecting/supplying stunning gas into the lower portion of the first chamber and at least one port for injecting/supplying stunning gas into the lower portion of the second chamber. - a first chamber equipped with an opening for introducing the poultry into said tunnel and with first conveying means for conveying the poultry through the first chamber to an opening for transferring the poultry, - a second chamber equipped with an opening for receiving the poultry and with second conveying means for conveying the poultry through the second chamber to an opening for evacuating the poultry from said tunnel, - an intermediate pass-through chamber separating the first and second chambers (100, 102) and communicating, at the ends thereof, with the transfer opening of the first chamber and the receiving opening of the second chamber, said intermediate chamber being equipped with third conveying means for conveying the poultry through the intermediate chamber, the first and second chambers (100, 102) being provided with stunning gas supply means ensuring an increase in the concentration of stunning gas from the opening for introducing the poultry into the tunnel to the opening for evacuating the poultry from said tunnel, said tunnel comprising: - a lower portion on which said first, second and third conveying means (100A, 101A, 102A) are mounted, and - an upper portion comprising an upper wall connecting side walls, said upper portion resting on the lower portion and being connected to lifting means intended to raise said upper portion relative to said lower portion, said facility being characterised in that it comprises at least one port (130, 131) for injecting/supplying stunning gas into the lower portion of the first chamber and at least one port for injecting/supplying stunning gas into the lower portion of the second chamber.