Cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were measured during normocarbia and during both moderate and severe hypocarbia. Eighteen newborn mongrel dogs, 1 to 7 days of age, were given pancuronium and ventilated with 70% N2O and 30% O2. The respirator was adjusted to achieve a PaCO2 of 15 torr, all subsequent changes to 25 and 40 torr were made by adjusting the inspired concentration of CO2. The sequence of PaCO2 levels was randomized. CBF was measured by microsphere technique and CMRO2 calculated as arterial-sagittal sinus O2 content difference times hemispheric blood flow. All measurements were made after 30 min at each PaCO2. Total CBF was reduced at a PaCO2 of 25 torr (p less than 0.001), further reduction in PaCO2 to 15 torr resulted in a significant decrease in total CBF (p less than 0.01) compared to 25 torr CO2. All regional cerebral blood flows were reduced at a PaCO2 of 25 torr (p less than 0.001), and most regional CBFs had further significant decreases in flow at a PaCO2 of 15 torr. CMRO2 was 1.28 +/- 0.47 ml/100 g/min at a PaCO2 of 40 torr and fell to 1.09 +/- 0.34 (p less than 0.05) and to 1.04 +/- 0.28 (p less than 0.025) ml/100 g/min at PaCO2 values of 25 and 15 torr, respectively. Cardiac output was calculated to be 169 +/- 71 ml/kg/min at a PaCO2 of 40 torr and fell to 135 +/- 27 (p less than 0.025) and to 127 +/- 36 (p less than 0.005) ml/kg/min at PaCO2 values of 25 and 15 torr, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)