Background:A sustained inflation (SI) facilitates lung aeration, but the most effective pressure and duration are unknown. We investigated the effect of gestational age (GA) and airway liquid volume on the required inflation pressure and SI duration.Methods:Rabbit kittens were delivered at 27, 29, and 30 d gestation, intubated and airway liquid was aspirated. Either no liquid (control) or 30 ml/kg of liquid was returned to the airways. Lung gas volumes were measured by plethysmography and phase-contrast X-ray-imaging. Starting at 22 cmH 2 O, airway pressure was increased until airflow commenced and pressure was then held constant. The SI was truncated when 20 ml/kg air had entered the lung and ventilation continued with intermittent positive pressure ventilation (iPPV).Results:Higher SI pressures and longer durations were required in 27-d kittens compared to 30-d kittens. During iPPV, 27-d kittens needed higher pressures and had lower functional residual capacity (FRC) compared to 30-d kittens. Adding lung liquid increased SI duration, reduced FRC, and increased resistance and pressures during iPPV in 29- and 30-d kittens.Conclusion:Immature kittens required higher starting pressures and longer SI durations to achieve a set inflation volume. Larger airway liquid volumes adversely affected lung function during iPPV in older but not young kittens.