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 PaCO2of 15 torr, all subsequent changes to 25 and 40 torr were made by adjusting the inspired concentration of CO2. The sequence of PaCO2levels was randomized. CBF was measured by microsphere technique and CMRO2calculated as arterial-sagittal sinus O2content difference times hemispheric blood flow. AH measurements were made after 30 min at each PaCO2. Total CBF was reduced at a PaCO2of 25 torr (P<0.001), further reduction in PaCO2to 15 torr resulted in a significant decrease in total CBF (P<0.01) compared to 25 torr CO2. All regional cerebral blood flows were reduced at a PaCO2of 25 torr (p<0.001), and most regional CBFs had further significant decreases in flow at a PaCO2of 15 torr. CMRO2was 1.28 ± 0.47 ml/ 100 g/min at a PaCO2of 40 torr and fell to 1.09 ± 0.34 (P<0.05) and to 1.04 ± 0.28 (P<0.025) ml/100 g/min at PaCO2values of 25 and 15 torr, respectively. Cardiac output was calculated to be 169 ± 71 ml/kg/min at a PaCO2of 40 torr and fell to 135 ± 27 (P<0.025) and to 127 ± 36 (P<0.005) ml/kg/min at PaCO2values of 25 and 15 torr, respectively. Regression analysis of the relationship between PaCO2and CBF for values of PaCO2between 10 and 50 torr was nonlinear (In CBF=a + b-PaCO2). A series of regression curves for regional CBF are presented with R values between 0.69 and 0.81 (P<0.001). It is concluded that hypocarbia to PaCO2values of 25 and 15 torr results in significant decreases in total as well as regional cerebral blood flows. CMRO2and cardiac output also show significant decreases at PaCO2values of both 25 and 15 torr compared to normocarbia. A nonlinear relationship is demonstrated between regional CBF and PaCO2between 10 and 50 torr.