Recent fetal lamb data have suggested that the pulmonary trunk (PT) region displays a reservoir function, and that a pharmacologically-induced fall in pulmonary vascular resistance (PVR) increases and redistributes diastolic discharge from this central pulmonary reservoir towards the lungs, thereby producing a positive diastolic offset in the pulmonary arterial (PA) blood flow profile. As a similar offset in PA flow characteristically occurs after birth, this study tested the hypotheses that 1) central pulmonary reservoir discharge is both redistributed towards the lungs and increased in magnitude during the birth transition and 2) discharge from this reservoir constitutes a major component of increased PA diastolic blood flow after birth. Six anaesthetized near-term fetal lambs were instrumented with PT, ductal and left PA transit-time flow probes, and aortic, PT and left atrial catheters. Hemodynamic data were recorded in fetuses and at regular intervals during 2 hours' mechanical ventilation following cesarean section delivery. Diastolic PA blood flow rose from near-zero in fetuses to 468±188 ml/min by 15 min (P<0.001). Central pulmonary reservoir discharge in fetuses (99±44 ml/min) passed primarily right-to-left across the ductus. However, this reservoir discharge redistributed entirely to the lungs by 1 min after birth, and then doubled to a peak of 214±167 ml/min at 15 min (P<0.001). Reservoir discharge subsequently stabilized at 151±60 ml/min at 30-120 min, which comprised 50% of diastolic and 20% of mean PA blood flow. These findings suggest that enhanced diastolic central pulmonary reservoir discharge plays a major role in supporting an increased pulmonary perfusion after birth.