Cerebral blood flow (CBF), CBF responses to changes in arterial CO2 tension, and cerebral metabolic rate for oxygen (CMRO2) were measured in newborn dogs, by means of a modification of the Kety and Schmidt technique employing 133Xe. Mongrel dogs of 1-7 days of age were paralyzed and passively ventilated with 70% N2O and 30% O2. CBF was derived by analysis of paired serial 20-microliter samples of arterial and of cerebral venous blood from the superior sagittal sinus. At an arterial PCO2 of 36.9 +/- 3.7 Torr and a mean arterial blood pressure of 62 +/- 10 Torr, CBF was 23 +/- 8 ml/min per 100 g. The arteriovenous oxygen content difference averaged 5.6 vol%, and CMRO2 was 1.13 +/- 0.30 ml O2/min per 100 g. CBF increased or decreased by 0.58 ml/min/100 g per Torr change in PCO2. Our results suggest that in the newborn, basal CBF and CBF responses to CO2 are considerably lower than in the adult and parallel the lower metabolic needs of the newborn brain.