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The development of medical countermeasures against the acute gastrointestinal subsyndrome of the acute radiation syndrome in humans requires well characterized and validated animal models. These models must adhere to the criteria of the U.S. Food and Drug Administration’s Animal Rule and consider the natural history and clinical context of the human radiation response and treatment in the nuclear terrorist scenario. The models must define the radiation dose- and time-dependent relationships for mortality and major signs of morbidity, including concurrent damage in other organs, such as the bone marrow, that may contribute to the overall mortality and morbidity. There are no such models of the gastrointestinal syndrome in response to total-body irradiation in the nonhuman primate. Herein, these parameters are defined for the rhesus macaque exposed to potentially lethal doses of radiation and administered medical management. Rhesus macaques (n = 69) were exposed bilaterally to 6 MV linear accelerator-derived photon total body irradiation to midline tissue (thorax) doses ranging from 10.0 to 14.0 Gy at 0.80 Gy min−1. Following irradiation, all animals were administered supportive care consisting of fluids, anti-emetics, anti-diarrheal medication, antibiotics, blood transfusions, analgesics, and nutrition. The primary endpoint was survival at 15 d post-irradiation. Secondary endpoints included indices of dehydration, diarrhea, weight loss, hematological parameters, cellular histology of the small and large intestine, and mean survival time of decedents. Mortality within the 15-d in vivo study defined the acute gastrointestinal syndrome and provided an LD30/15 of 10.76 Gy, LD50/15 of 11.33 Gy, and an LD70/15 of 11.90 Gy. Intestinal crypt and villus loss were dose- and time-dependent with an apparent nadir 7 d post-irradiation and recovery noted thereafter. Severe myelosuppression and thrombocytopenia were noted in all animals, requiring the administration of antibiotics and blood transfusions. The model defines the dose response relationship and time course of acute gastrointestinal syndrome-induced morbidity and mortality in the rhesus macaque.