At birth, the initiation of pulmonary gas exchange is dependent on air entry into the lungs and recent evidence indicates that pressures generated by inspiration may be involved. We have used simultaneous plethysmography and phase contrast X-ray imaging to investigate the contribution of inspiration and expiratory braking maneuvers (EBMs) to lung aeration and the formation of a functional residual capacity (FRC) after birth. Near term rabbit pups (n=26) were delivered by caesarean section, placed in a water plethysmograph and imaged during the initiation of spontaneous breathing. Breath-by-breath changes in lung gas volumes were measured using plethysmography and visualized using phase contrast X-ray imaging. Pups rapidly (1-5 breaths) generate a FRC (16.2±1.2ml/kg) by inhaling a greater volume than they expire (by 2.9±0.4 mL/kg/breath over first 5 breaths). As a result, 94.8±1.4% of lung aeration occurred during inspiration over multiple breaths. The incidence of EBMs were rare early during lung aeration, with most (>80%) occurring after >80% of max FRC was achieved. Although EBMs were associated with an overall increase in FRC, 34.8±5.3% of EBMs were associated with a decrease in FRC. We conclude that lung aeration is predominantly achieved by inspiratory efforts and that EBMs help to maintain FRC, following its formation.