Brazilian poultry facilities offer greater possibility for natural ventilation as compared to facilities placed in colder countries. However, the information about gas concentrations inside these buildings is limited. These gas concentrations can be harmful to workers and animal health. Thus, the objective of this work was monitoring the gas concentrations most commonly found inside poultry production environment and the influence of three different heating systems in the gas concentrations. The research was developed at nine commercial poultry facilities, where CO, C0 2 and NH 3 concentrations inside the production environment were evaluated. It can be concluded that despite instantaneous concentrations of NH 3 and CO being found at levels above recommended by literature, the health of animals was not affected, which could be shown by the animal performance values.