Abstract: A method for monitoring gas stunning of birds, where groups of birds are conveyed into and out of a stunning zone (1), and where the concentration of the stunning gas in the stunning zone is measured and adjusted for optimal stunning. The gas concentration is measured at a plurality of measuring positions in the stunning zone and a visual inspection of the birds is performed at two or more of the measuring positions using a visual inspection unit (76), such as a camera, which is moved into the stunning zone together with the group of birds. The groups of birds are advantageously kept in crates (3). The visual inspection unit may be part of a monitoring unit (71) further including a gas sensor (75), said monitoring unit being used for simultaneous measuring of the concentration of the stunning gas and inspection the birds at two or more monitoring positions.