UP to now there are at least two plethysmographic methods to measure tidal volume including whole body plethysmograph and double-chamber plethysmograph. But there is one problem that "tidal volume" from this way is thoracic inflating volume. Is tidal volume equal to thoracic inflating volume? In this experiment we use different barometric plethysmograph to measure the pressure changes inside the chambers，furthermore we calculate tidal volume and thoracic inflating volume of mice，and evaluate their differences.