1. Poultry on farms are sometimes required to be killed in an emergency, such as during a disease epidemic, yet none of the available methods are ideal. Whole-house carbon dioxide (CO(2)) administration has practical advantages, but gives rise to welfare concerns. 2. The study measured the body temperature, respiration, cardiac and brain activity (EEG) responses of 10 adult hens placed in tiered cages in a deep pit house while the entire flock (28,000 end-of-lay hens) was killed with CO(2). Video and thermographic images were also recorded. Liquid CO(2) was injected into the building producing a gaseous concentration of 45% within 19 min. 3. Those hens nearest the gas delivery site showed delayed respiratory, cardiac and EEG responses compared with those at more distant locations. Although sub-zero temperatures were recorded in the immediate vicinity of some birds, body temperatures indicated that they did not die of hypothermia. 4. EEG characteristics strongly associated with unconsciousness were used to determine an unequivocal time to loss of consciousness; this ranged from 6·0 to 10·5 (average 7·8) min after onset of gas injection. Distinctive cardiac and respiratory responses were seen following gas exposure; in particular, birds responded to inhalation of CO(2) by deep breathing. 5. The primary welfare concern is the duration of unpleasant respiratory effects, such as deep breathing, while the birds were substantively conscious. However, the concentration of CO(2) to which the birds were exposed while conscious would not have stimulated nasal and oral nociceptors. Time to death varied between 12·0 and 22·1 min after gas delivery.