OBJECTIVE Neutrophils may play an important role in cerebral ischaemia. We investigated whether neutrophil activation can be detected in cerebral venous blood during the mild cerebral hypoxia and reperfusion that occurs during carotid cross clamping and declamping for endarterectomy. DESIGN, SETTING AND MATERIALS: The ipsilateral jugular bulb was cannulated at operation in 16 patients undergoing carotid endarterectomy. Blood was taken immediately prior to and 30 seconds following internal carotid cross clamping; then immediately prior to, 30 s and 2 m following declamping. Blood was also taken from a peripheral vein in the foot. Intracerebral oxygen saturation (CsO2) was measured continuously by near infrared cerebral spectroscopy. Neutrophil activation was measured by flow cytometric detection of fluorescence to hydrogen peroxide in unstimulated cells and phorbol myristate acetate stimulated cells, and expressed as mean fluorescent intensity (MFI). METHODS Neutrophil activation and cerebral oxygenation. RESULTS CsO2 fell from 68% (95% Confidence interval 64%-72%) to 63% (59%-68%) following carotid cross clamping (p < 0.05, repeated measures analysis of variance). This recovered slightly during the cross clamp period to 64% but only returned to preclamp levels following declamping (p > 0.05). Neutrophil hydrogen peroxide generation by stimulated neutrophils rose significantly from 0.79 mean fluorescent intensity (0.53-1.19) to 1.46 (0.98-2.20) but no there was no further rise following cross clamp release. There was no significant neutrophil activation in the peripheral samples. CONCLUSIONS These results indicate that even mild cerebral hypoxia is associated with priming of neutrophils in cerebral venous blood.