The indoor air quality in schools is very important for health and learning abilities of children. The primary indoor CO2 source in classrooms is the respiration of school building occupants. Also, CO2 comes from outside as a result of fossil fuels combustion. CO2 concentration depends on a ventilation rate, size of the classroom, number of occupants and their activity and time they spend in school building. Unfortunately, ventilation rates in schools were not often measured, even in cases when inadequate ventilation caused pupils' health problems and their absence from school. The increase in indoor CO2 concentration above the outdoor concentration is considered as a good surrogate for the indoor concentrations of bio effluents. This paper presents the research of ventilation rates in five naturally ventilated schools in urban and rural areas in Serbia during the heating season. CO2 concentrations were measured outdoor and in three classrooms for five working days, continually. Ventilation rates are calculated based on measured concentrations of CO2. The results have shown that classrooms in Serbian schools have inadequate ventilation during the heating period. Mean value of carbon dioxide concentration has often been exceeding 1000 ppm. (C) 2014 Elsevier Ltd. All rights reserved.