In order to study gaseous dispersion in the central airways during spontaneous breathing, the results of the computer simulation of a mathematical model were compared with those of an appropriate in vivo test performed on normal subjects. The model is governed by a gas transport equation-with a convection and dispersion term-solved for Weibel geometry by a linite element method. The numerical computation for this equation was tested by the analytical solution in a particular geometric situation. For each trial (injection of a 3-ml argon bolus in the latter part of a quiet inspiration), a simulation was performed, using the inspiratory bolus as the nonxero limit condition (input signal); the value of the mixing coefficient 9 was optimixed so as to fit the experimental data. Whatever the value of 9, no similarity could be found when the bolus was injected in front of the lips. In one volunteer the injection was performed through an endotracheal tube: computer and experimental results are much closer. It is concluded that (1) the upper airways play a specific role in gas transport (2) the model should be improved by allowing variations of 9 according to the depth in the airways and the gas velocity. * Supported by a grant from INSERM (no 77.5.027.5) and a grant from Mini.st&re de l'environnement et du cadre de vie. (78-87).