Nitrogen washout measurements and blood-gas analyses were made on 32 newborn infants with severe RDS at continuous positive airway pressures (CPAP) of 5, 10, and 15 cm H2O. Increases in airway pressure resulted in significant increases in PaO2 and functional residual capacity (FRC). It also produced significant decreases in alveolar turnover rates of the "fast" and "slow" alveolar spaces of a two-space lung model. Changes in CPAP did not significantly affect the distribution of ventilation. The changes in PaO2, due to changes in CPAP, did not correlate well with changes in FRC/wt nor with changes in alveolar turnover rates. Thus, the effects of increasing CPAP on PaO2 were not simply due to increases in FRC. The changes in PaO2 are due to a complex relationship between changes in FRC, alveolar turnover rates, and to other alterations in cardiopulmonary function that are yet to be fully understood.