The inspired oxygen concentration from pressure limited ventilators with a Venturi air mix rises steeply when the inflating pressure exceeds approximately 8 cm. A mean oxygen concentration of 82% was obtained when a lung model (compliance 0.032 liter/cm) was ventilated with a tidal volume of 0.8 liter. Concentrations of 93% and 61% were delivered when a main line nebulizer was incorporated and driven by a flow of 8 liters of oxygen or air, respectively. Thirty percent to 50% oxygen was obtained only by powering the ventilator by compressed air, and adding oxygen at increasing flows via the main line nebulizer. For volume limited ventilators a graph is shown to predict inspired oxygen. Its validity is dependent upon accurate stroke volume calibration and flowmeters, particularly with the small volumes and flows used in children.