An investigation was carried out into the relationship between the inflation pressures of normally compliant lungs and the airways pressure necessary to produce the insufflation of gas to the stomach. This relationship was examined during manual ventilation with a mask, using a rapid sequence induction technique. In the absence of cricoid pressure the lungs of all the patients could be ventilated "gently" satisfactorily by hand without gas entering the stomach. In only half of the patients could gas be redirected to the stomach when maximal inflation pressures were generated. It was not possible to cause gas to enter the stomach in any patient with a patent airway when cricoid pressure was applied.