The use of individually ventilated cage (IVC) systems has become more common worldwide. The various systems are becoming more and more sealed in order to protect the animals against infections and the staff against allergens; which, however, may lead to problematic CO2 concentrations, if the cages are left unventilated. In this study it is shown that, depending on how tight the cage is and the number of animals housed in each cage, CO2 inside the cage within 2 h will increase to levels of between 2 and 8%.