The influence of CPR on the blood-brain barrier (BBB) in dogs was studied by fluorescence microscopy using Evans blue as a BBB tracer.Three of 9 control dogs. whose blood pressure (BP) was increased by more than 130 mm Hg, showed dye extravasation (BBB impairment) into brain tissue, but no extravasation was seen in the other 6 controls whose BP increase was less than 110 mm Hg.In 11 experimental animals, extravasation of Evans blue occurred in 8 of 9 resuscitated dogs after several minutes of cardiac arrest and in 1 of 2 nonresuscitated dogs. In the resuscitated dogs, the maximum systolic BP (peak BP) after restoration of ventricular function ranged from −40 to +210 mm Hg compared to control values. They showed all grades of dye extravasation without significant correlation between the magnitude of BP increase and the severity of dye extravasation.The authors conclude that BBB disruption usually occurs during CPR and causes plasma protein extravasation into brain tissue.Arterial hypertension is a causative factor for BBB disruption, as was seen in the control dogs. In resuscitation, restoration of BP sufficient to overcome intracranial pressure and maintain cerebral blood flow may be high enough to induce BBB disruption because cerebral vessels are fully dilated and functionally impaired due to longstanding anoxia. The rapidity of BP increases after restoration of ventricular function could be another factor which might contribute to BBB impairment.