Background: At birth, the transition to newborn life is triggered by lung aeration, which stimulates a large increase in pulmonary blood flow (PBF). Current theories predict that the increase in PBF is spatially related to ventilated lung regions as they aerate after birth. Using simultaneous phase contrast X-ray imaging and angiography we investigated the spatial relationships between lung aeration and the increase in PBF after birth. Methods and Results: Six near-term (30d gestation) rabbits were delivered by caesarean section, intubated and an intravenous catheter inserted, before they were positioned for X-ray imaging. During imaging, iodine was injected before ventilation onset, after ventilation of the right lung only and after ventilation of both lungs. Unilateral ventilation increased iodine levels entering both left and right pulmonary arteries (PAs) and significantly increased heart rate, iodine ejection per beat, diameters of both left and right PAs, number of visible vessels in both lungs. Within the 6th intercostal space, the mean grey level (relative measure of iodine level) increased from 68.3±11.6 and 70.3±7.5 %.sec to 136.3±22.6 and 136.3±23.7 %.sec in the left and right PAs, respectively. No differences were observed between vessels in the left and right lungs, despite the left lung not initially being ventilated. Conclusion: The increase in PBF at birth is not spatially related to lung aeration allowing a large ventilation/perfusion mismatch, or pulmonary shunting, to occur in the partially aerated lung at birth.