Background: Carbon monoxide (CO) is the leading cause of poisoning morbidity and mortality in the United States. Standard treatment includes supplemental oxygen and supportive care. The utility of hyperbaric oxygen (HBO) therapy has been challenged by a recent Cochrane review. Hypothermia may mitigate delayed neurotoxic effects after CO poisoning as it is effective in cardiac arrest patients with similar neuropathology. Objectives: To develop a rat model of acute and delayed severe CO toxicity as measured by behavioral deficits and cell necrosis in post-sacrifice brain tissue. Methods: A total of 28 rats were used for model development; variable concentrations of CO and exposure times were compared to achieve severe toxicity. For the protocol, six senescent Long Evans rats were exposed to 2,000 ppm of CO for 20 minutes then 1,500 ppm for 160 minutes, followed by three successive dives at 30,000 ppm with an endpoint of apnea or seizure; there was a brief interlude between dives for recovery. A modified Katz assessment tool was used to assess behavior at baseline and 2 hours, 1 day, and 1, 2, 3, 4, 5, and 6 weeks post-exposure. Following this, the brains were transcardially fixed with formalin, and 5 lm sagittal slices were embedded in paraffin and stained with hematoxylin and eosin. A pathologist quantified the percentage of necrotic cells in the cortex, hippocampus (pyramidal cells), caudoputamen, cerebellum (Purkinje cells), dentate gyrus, and thalamus of each brain to the nearest 10% from 10 randomly selected high power fields (400x). Results: Measured carboxyhemoglobin concentrations immediately following the protocol were 55–70%. Both acute and delayed behavioral deficits were demonstrated in this rat model of CO toxicity, which parallels the neurocognitive deficit pattern observed in humans (see figure). Similar to prior studies, pathologic analysis of brain tissue demonstrated the highest percentage of necrotic cells in the cortex, pyramidal cells, and cerebellum. The collected data are summarized in the table. Conclusion: We have developed an animal model of severe CO toxicity evidenced by behavioral deficits and neuronal necrosis. Future efforts will compare neurologic outcomes in severely CO poisoned rats treated with hypothermia and 100% inspired O2 versus HBO to normothermic controls treated with 100% inspired O2.