Cerebral blood flow was measured with the133Xenon clearance method during short-lasting (20 minutes) and more prolonged (90 minutes) infusions of Na2CO3solutions in anesthetized cats under controlled ventilation. The infusion protocol was regulated so as to produce a given increase in the plasma [HCO3−] in the first 15 minutes, followed by a constant high plasma level for the rest of the infusion period. A high Paco3level was induced before and at the end of the infusion, when prolonged infusions were made. The results indicate that, in acute experiments (20 minutes), an increase in plasma [HCO3−] of 14 mEq/l does not influence CBF. During more prolonged infusions (90 minutes), an increase of 12 mEq/l produces a reduction of CBF and an increase in the CSF [HCO3−]. These changes are more pronounced when the increase in plasma [HCO3−] is more marked (18 mEq/l).