Glutamate, the major central nervous system neurotransmitter, may have potent neurotoxic activity under conditions of metabolic stress. By receptor autoradiography, we have demonstrated that brain regions most vulnerable to injury during prolonged hypothermic circulatory arrest have the highest density of glutamate receptors. To test the hypothesis that such injury could be mediated by glutamate excitotoxicity, we used dizocilpine (MK-801), a selective N-methyl-D-aspartate-glutamate receptor antagonist in a canine survival model of hypothermic circulatory arrest. Eighteen male dogs (20 to 25 kg) were supported by closed-chest cardiopulmonary bypass, subjected to 2 hours of hypothermic circulatory arrest at 18° C, and rewarmed on cardiopulmonary bypass. All were mechanically ventilated and monitored for 20 hours before extubation and survived for 3 days. Group A dogs (n = 9) received a prearrest intravenous bolus of dizocilpine (0.75 mg/kg) followed by continuous infusion (75 μg/kg per hour), resulting in electroencephalographic silence. Dizocilpine was weaned before extubation. Group B dogs received vehicle only. According to a species-specific behavior scale that yielded a neurologic deficit score ranging from 0 (normal) to 500 (brain dead), all animals were neurologically assessed every 12 hours. After the dogs were killed at 72 hours, brains were examined by receptor autoradiography and histologically for patterns of selective neuronal necrosis; they were scored blindly from 0 (normal) to 100 (severe injury). Group A dogs had better neurologic function than group B (neurologic deficit score 21 ± 15 versus 192 ± 40, p < 0.001) and had less neuronal injury (7.3 ± 3 versus 48.3 ± 9, p < 0.0001). Densitometric receptor autoradiography revealed preservation of neuronal N-methyl-D-aspartate-glutamate receptor expression in group A only. These results represent the first direct evidence of a role for glutamate excitotoxicity in the development of hypothermic circulatory arrest-induced brain injury and suggest that selective glutamate receptor antagonists may have a neuroprotective capacity in prolonged periods of hypothermic circulatory arrest.