We determined the effects of three components of the birth process on central blood flow patterns in fetal sheep. We instrumented 16 fetal sheep at 133.0 +/- 1.2 days gestation, inserting various intravascular catheters, intubating the trachea, and placing an inflatable balloon around the umbilical cord. After 2-3 days, we determined central blood flow patterns using radionuclide-labeled microspheres under control conditions, during positive pressure ventilation without oxygenation, during ventilation with 100% O2, and after umbilical cord occlusion. The foramen ovale right to left shunt was essentially abolished, decreasing from 102 +/- 48 to 66 +/- 40 ml/min/kg with ventilation, and to only 13 +/- 10 ml/min/kg with oxygenation. The ductus arteriosus right to left shunt decreased progressively, from a control level of 224 +/- 64 to 6 +/- 10 ml/min/kg after umbilical cord occlusion. A ductus arteriosus left to right shunt appeared with oxygenation (41 +/- 26 ml/min/kg) and increased to 65 +/- 43 ml/min/kg after cord occlusion. Left ventricular output increased progressively as a percentage of combined ventricular output (from a control value of 34.8 to 59.5% after cord occlusion), and increased absolutely with ventilation (from 134 +/- 44 to 211 +/- 87 ml/min/kg). However, right ventricular output decreased (from a control value of 258 +/- 75 to 144 +/- 36 ml/min/kg after cord occlusion) so that combined ventricular output did not change. Although the transition from the fetal to neonatal circulatory pattern was accomplished by simulating these three components of the birth process, none is alone responsible for the large increase in combined ventricular output normally seen at birth.