Background : Bag and mask devices are used frequently to provide patients with positive-pressure- assisted ventilation. One of the disadvantages is the fact that they do not deliver high concentrations of oxygen without special adaptors or attention to technique. In order to investigate the variables affecting oxygen delivery, we designed a study to determine the fractions of delivered oxygen (FDO2) under varying ventilating techniques and conditions. Methods : We designed special wooden box, in which the Laerdal resuscitator bag had been. We measured the fractions of delivered oxygen with or without reservoir bag in various tidal volumes, respiration rates and oxygen flows. Results : Without reservoir bag, the fractions of delivered oxygen were increased up to only 73% in spite of 15 l/min oxygen flow. With reservoir bag, the fractions of delivered oxygen were increased up to nearly 96% in 5∼7.5 l/min oxygen flow. Conclusions : While using the Laerdal resuscitator bag, it is desirable to adapt reservoir bag and supply 5 l/min oxygen in conventional ventilation and 7.5 l/min in hyperventilation minimally for higher fraction of delivered oxygen. (Korean J Anesthesiol 1999; 36: 481∼485)