We examined the influence of the following parameters in determining the FiO2 delivered to a pediatric lung model using the mouth-to-mask method of resuscitation: rate of ventilation, inspiratory tidal volumes, and supplemental oxygen flow. With a ventilator rate of 20/min and tidal volumes (Vt) < or = 100 mL, an FiO2 of approximately .50 was observed with a supplemental oxygen flow of 5 L/min. Increasing the supplemental oxygen flow to 15 L/m did not appreciably increase the FiO2 (FiO2 = .53 versus FiO2 = .60, respectively), but did cause a significant and unintended increase in Vt. Similar results were noted with a ventilator rate of 12/min and Vt < or = 100 mL (FiO2 = .68 versus FiO2 = .73, respectively). We also observed a potentially hazardous situation involving the positioning of the supplemental oxygen port that might result in high inspiratory pressures (stacking of breaths) to the pediatric patient. We believe additional testing is warranted prior to widespread use of this device in children.