Gas injection systems used in several techniques of ventilation cause increases in airway pressure and physiological changes which are frequently overlooked. The momentum flux theory describes such phenomena most appropriately. We have defined and measured the characteristics of such increases in airway pressure, using a lateral tracheal injection system which has been described previously. In such a system, wall friction is a major source of loss in jet momentum flux, in contrast with the changes in axis-symmetrical systems. This process results in a potentially beneficial increase in airway pressure and in greater mixing, which might be clinically useful.