The effect of CO2 on the cerebral circulation was assessed 24 hours after induction of unilateral brain infarction performed in the rat by injecting radioactive calibrated 50 microns microspheres into the internal carotid artery. The intracerebral distribution of microspheres and regional cerebral blood flow were measured bilaterally in 8 brain regions. In control rats, increase in arterial pCO2 to about 80 mm Hg resulted in 30 to 100% increase in flow according to the area. Cerebral blood flow was also enhanced in the embolized rats, the basal values being multiplied by a factor 1.7 in the embolized hemisphere and by a factor 1.8 in the contralateral hemisphere. These results do not provide evidence for the existence of a "steal" phenomenon between the non infarcted and infarcted hemispheres.