Small children and neonates present special problems when they are to have automatic ventilation. These derive from their small tidal volumes, rapid respiratory rates, small inspiratory flow rates and low compliance pressures. In genral, automatic ventilators designed for adults do not work satisfactorily for small subjects. Either the flow is too large, the repiratory rate too slow, the tidal volume too large or the cycling pressure too high. Theoretical concepts were tested by applying various ventilatos to model lungs representative of different ages of children. Clinical tests support the theoretical and experimental results.