Exhalation of carbon dioxide (CO2) by pigs was investigated under field conditions in a mechanically ventilated commercial fattening house. The tranquil CO2 exhalation rate (TCER) by pigs was defined and methodology was developed to study it. The experiments were conducted by moving groups of pigs in and out of one of the compartments in the house and comparing differences of measured CO2 production rates. The measured TCERs ranged from 41.5 to 73.9 g CO2 h(-1) per pig for pigs from 32 to 105 kg. When pigs were very active, the CO2 exhalation rate could be about 200% of the TCER but did not last for long time. A TCER mathematical model was developed based on 4 sets of experiments. It calculated the CO2 exhalation by a pig at tranquil time as a function of its weight. Daily mean CO2 exhalation rate (CER) by a pig was about 110% of the TCER, The TCER/CER model related the CO2 exhalation to some aspects of pigs' behaviours and was the first reported model developed with direct measurement of CO2 production rates. Five models of CO2 exhalation in available literature were reviewed and the CER model was compared with them. There was a clear disparity among these models. The average CO2 exhalation rate calculated with the "Ouwerkerk Model" was about three times as that obtained by the "Anderson Model" for pigs from 35 to 120 kg. The CER model produced the same CO2 exhalation rate as the "Ouwerkerk Model" for a pig of 35 kg and a close rate to the "Klooster Model" for a gig of 85 kg, (C) 1999 Elsevier Science Ltd. All rights reserved.