Regional cerebral blood flow (rCBF) was measured by the123Xe intra-arterial injection method with 14 external scintillation detectors in 21 operations for stenosis of the internal carotid artery. rCBF and internal carotid arterial pressure distal to the stenosis were measured simultaneously before and after endarterectomy and during two test occlusions of the earotid artery at different Paco2values. In one group measurements during test occlusion were made at hypo- and hypercap-nia; in the other, at normo- and hypercapnia. During hypocapnia (mean Paco230 mm Hg) and normocapnia (mean Paco238 mm Hg) autorgu-lation was partially preserved, while during hypercapnia (mean Paco257 mm Hg) rCBF was linearly related to internal earotid arterial pressure. In both groups hypercapnia decreased internal erotid arterial occlusion pressures, but average mean rCBF values increased. In five patients, however, paradoxical rCBF responses to elevation of Paco2were observed, either focally or in the whole hemisphere. It is concluded that hypercapnia may reduce flow to poorly perfused regions.