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Changes in the somatosensory evoked potentials (SEPs) and spontaneous electroencephalogram (EEG) in hens were investigated during stunning with a mixture of 30 per cent carbon dioxide in argon with 5 per cent residual oxygen. The results showed that the SEPs were lost on average in 17 seconds (maximum 28 seconds), which is similar to the 19 seconds (maximum 32 seconds) reported while stunning hens with a mixture of 30 per cent carbon dioxide in argon with 2 per cent residual oxygen. The spontaneous EEG showed suppression and a quiescent phase at 14 and 58 seconds, respectively. It is concluded that a mixture of 30 per cent carbon dioxide in argon with 2 per cent residual oxygen would be ideally suited for batch stunning chickens and any inadvertent increase in the residual oxygen level up to 5 per cent in the stunning atmosphere would not lead to inadequate stunning or recovery of consciousness before neck cutting.