Profound hypothermia induced with cardiopulmonary bypass has a protective effect on spinal cord function during operations on the thoracoabdominal aorta. The mechanism of this protection remains unknown. It has been proposed that the release of excitatory amino acids in the extracellular space plays a causal role in irreversible neuronal damage. We investigated the changes in extracellular neurotransmitter amino acid concentrations with the use of in vivo microdialysis in a swine model of spinal cord ischemia. All animals underwent left thoracotomy and right atrium-femoral artery cardiopulmonary bypass with additional aortic arch perfusion. Lumbar laminectomies were then done and microdialysis probes were inserted stereotactically in the anterior horn of the second and fourth segments of the lumbar spinal cord. The probes were perfused with artificial cerebrospinal fluid at a rate of 2 microliters/min and 15-minute samples were assayed by high-performance liquid chromatography. Group 1 animals (n = 6) underwent aortic clamping distal to the left subclavian artery and proximal to the renal arteries for 60 minutes at normothermia (37 degrees C) and group 2 animals (n = 5) were cooled to a rectal temperature of 20 degrees C before application of aortic clamps, maintained at this level during cardiopulmonary bypass until the aorta was unclamped, and then slowly rewarmed to 37 degrees C. Seven amino acids were studied, including two excitatory neurotransmitters (glutamate and aspartate) and five putative inhibitory neurotransmitters (glycine, gamma-aminobutyric acid, serine, adenosine, and taurine). Glutamate exhibited a threefold increase in extracellular concentration during normothermic ischemia compared with baseline values and remained elevated until 60 minutes after reperfusion. The increase in aspartate concentration was not significant. The extracellular concentrations of glycine and gamma-aminobutyric acid also increased significantly during ischemia and reperfusion. Hypothermia uniformly prevented the release of amino acids in the extracellular space. Glutamate levels remained significantly decreased even after rewarming to normothermia whereas glycine levels returned to baseline values. These results are consistent with a role for excitatory amino acids in the production of ischemic spinal cord injury and suggest that the mechanism of hypothermic protection may be related to decreased release of these amino acids in the ischemic spinal cord.