Continuous positive airway pressure (CPAP) was used to support the ventilation of infants less than 3 months of age who had undergone thoractomy for cardiovascular surgery. The functional residual capacity, which was approximately 30 per cent of predicted at zero CPAP, increased 35 per cent in cyanotic and 33 per cent in acyanotic infants with the application of 5 mm Hg pressure. Increasing airway pressure from zero to 5 mm Hg increased PaO2 4 per cent in cyanotic and 13 per cent in acyanotic infants. There was no change in heart rate, respiratory rate, mean arterial pressure, pH or PaC02 under similar circumstances, but central venous pressure increased 1.5 mm Hg in cyanotic and 0.8 mm Hg in acyanotic infants.