At birth, the cardiovascular system changes dramatically; arterial blood pressure, heart rate, and cardiac output increase, and blood flow distribution undergoes regional changes. To determine whether these changes are related to certain events occurring at birth, we studied 18 chronically instrumented fetal sheep at 133-138 days gestational age. We measured fetal vascular pressures and heart rate and injected radionuclide-labeled microspheres to determine combined ventricular output and its distribution. Rhythmic ventilation of the fetuses with a gas mixture that produced no change in arterial blood gases decreased heart rate slightly from 173 +/- 25 to 152 +/- 16 beats/min, but changed neither mean arterial blood pressure nor combined ventricular output. Ventilation with oxygen decreased mean descending aortic and pulmonary arterial pressures, but had no significant effect on heart rate or combined ventricular output; subsequent umbilical cord occlusion had no significant effect on these variables. There was a redistribution of blood flow, however. Ventilation alone increased pulmonary blood flow from 159 +/- 68 to 641 +/- 513 ml.min-1.100 g wet weight-1, and oxygenation increased it further to 1040 +/- 426. Ventilation alone and ventilation with O2 decreased blood flow to most other fetal organs, notably the adrenal glands, brain, and heart, in a manner related to changes in arterial oxygen tension. Occlusion of the umbilical cord increased blood flow to the adrenal glands and brown fat, and also to the liver from the portal vein and hepatic artery. The changes in regional blood flow initiated by ventilation, oxygenation, and umbilical cord occlusion are similar to those occurring at birth and may play an important role in postnatal adaptation.