Unexplained anatomic and physiologic factors account for the unacceptably high rate of paraplegia/paresis after thoracoabdominal aortic reconstruction. We assessed the neurologic significance of patent internal mammary arteries (!MAs) in a hovel rat model of aortic clamping in which the aortic origins of the intercostal arteries (ICAs) were occluded. Methods: Twenty anesthetized, intubated, and halothane-ventilated adult male rats had catheters placed in the carotid and femoral arteries. ICAs arising from the aorta were divided at their origins through a left thoracotomy. IMAs were either divided (IMA-OUT, n = lO) or left intact (IMA-IN, n = lO). Proximal and distal descending aortic clamps were placed for 7 minutes. A neurologic deficit score (NDS) was assigned at 1, 4, 18, 24, and 48 hours by use of an established scoring system. Results: The mean IMA-IN NDS was statistically better than the mean IMA-OUT NDS at 1, 4, 18, and 24 hours with P = 0.0005, 0.0014, 0.0098, and 0.0151, respectively. Moreover, the mortality rate in the IMA-OUT group was statistically greater than in the IMA-IN group (p = 0.0036). Conclusion: In this model, patent lMAs prevent paraplegia when the ICAs are occluded at their aortic origin and the aorta is clamped for 7 minutes.