According to our present knowledge, mainly due to O2 absorption the pressure continuously decreases in the normal middle ear, which is equalized by time to time opening of the Eustachian tube, so that air flows across the tube. OBJECTIVE The aim of the author is to revise this classic theory on the basis of his own research and the results of the last decades. The basic question is whether the pressure increases or decreases in the normal middle ear between two swallowings? METHODS Middle ear pressure change was examined in 32 ears of 16 healthy children in the swallowing pause (between two swallowings) with tympanometry. The patients were 14 +/- 5.2 years old. RESULTS According to the results of the examinations, the middle ear pressure increases between two swallowings in healthy individuals, which is statistically significant (p = 0.028). CONCLUSIONS In his opinion this can only be explained by one factor, and that is CO2 diffusion. The pressure gradient of CO2 that is directed towards the middle ear cavity is upheld by the Eustachian tube - every time the tube opens the middle ear loses CO2 (the partial pressure of CO2 in the middle ear is higher, in the atmospheric air is very low), so after tube is closed, CO2 diffuses from surrounding tissue to the middle ear and the pressure increases. This CO2 loss can take place if we consider that gases not only flow according to their composition percent, but also diffuse according to their partial pressure difference through the tube.