Continuous positive airway pressure (CPAP) is often used to provide noninvasive respiratory support in infants with Respiratory distress syndrome. The recruitment of atelectatic lung and appropriate lung volume maintenance are vital to the success of CPAP treatment. The noisy pressure waveform of bubble CPAP superimposed on pressure fluctuations as a result of spontaneous breathing may promote airway opening events as a result of stochastic resonance. The magnitude and the frequency of the superimposed noise are critical to this process. We hypothesized that the applied bias flow and mechanical properties of the lung would influence the magnitude and the frequency content of the noise transmitted to the lung. The effect of varying bias flow (6-10 L/min) and lung compliance (0.1-1.5 mL/cm H(2)O) on the mean, range, and frequency content of the pressure fluctuations at the airway opening and within the lung was evaluated in an in vitro model lung. Increasing bias flow increased the mean and the magnitude of pressure oscillations at the airway opening and in the lung model. Decreasing compliance of the lung model increased the magnitude and the frequency content of pressure oscillations in the model lung. Lung mechanics and applied flow influence the magnitude of the noise superimposed on the transmitted pressure waveform and may influence lung volume recruitment in bubble CPAP.