Exposure to a high concentration of carbon dioxide (CO2) is a widely used method to stun pigs. During inhalation, loss of consciousness is not immediate and there is a period of time when the animals can suffer from suffocation or distress. The aim of this study was to assess the brain activity of pigs with an index of consciousness (IoC) given by the electroencephalogram (EEG) during the exposure to 90%CO2 and to know how this is related to different behaviours (gasping, vocalisations, muscular excitation) and reflexes (loss of posture). IoC view®(Morpheus Medical) is a device that assesses the consciousness by an algorithm that analyzes the EEG and gives an index from 0 (unconscious) to 100 (conscious). Ten pigs of 93±1.8kg weight were individually exposed to an atmosphere of 90%CO2 during 2 min by descending into a crate of 3m depth. The IoC of each pig was recorded during 5 min before the exposure to assess the basal values, during the 2min of exposure to assess the loss of consciousness and during 5 min after the exposure to assess the recovery. The data were analyzed using Mixed models of SAS. The IoC value (56) was significantly lower (p<0.05) than basal level at 87s from the beginning of the exposure to the gas. During the induction, 90% of the animals showed gasping at 22.1±0.82s and all the animals lost balance at 23.8±1.53s; 90% of pigs showed muscular excitation at 24.3±1.50s, 50% vocalised at 33.0±2.41s and all of them showed gagging at 38.3±0.87s. IoC values did not reach the basal level during the five min after exposure. According to the results, loss of balance could not be taken as an indicator of loss of consciousness in pigs and the beginning of vocalisations and muscular excitation could occur when the animal is still conscious.