In order to determine the relative roles of O2tension and content, CO2tension, hydrogen ion concentration, arterial blood pressure, and cardiac output in the regulation of fetal cerebral blood flow (CBF), we used radioactively labeled microspheres to measure flow to 20 major brain regions in 24 chronically catheterized fetal lambs. We continually monitored fetal heart rate and blood pressure, and periodically measured arterial Po2, Pco2, pH, and hematocrit. In addition to CBF measurements during control periods, we measured CBF during: 1) hypoxia (O2content<6 mI·dl-1; O2tension<15 torr) induced by having the ewe breathe a gas mixture with low O2concentration, 2) hypercapnia (Pco2>50 torr) induced by increasing the maternal inspired CO2, 3) acidosis and alkalosis (7.60>pH>6.60) induced by infusing lactic acid or bicarbonate into the fetus, and 4) hypotension (blood pressure<35 mm Hg) and hypertension (blood pressure>55 mm Hg) induced by rapidly phlebotomizing or transfusing the fetus. We used multiple regression analysis and analysis of covariance to examine the dependence of total cerebral blood flow on arterial O2tension and content, CO2tension, pH, blood pressure, and cardiac output. The results demonstrated that 1) fetal CBF increased linearly as oxygen tension or content decreased and a hierarchy of responsivity occurred (brainstem>subcortex and cortex), 2) fetal CBF increased as carbon dioxide tension increased with a different hierarchy of responsivity (brainstem>subcortex>cortex), and 3) autoregulation of fetal CBF over a wide range of blood pressure or cardiac output was maintained for both total CBF and the various brain regions examined.