It has recently been demonstrated that N-methyl-D-aspartate antagonists limit tissue damage after spinal cord trauma, implicating excitatory amino acids in the secondary injury response. To determine whether spinal cord trauma alters the concentrations of extracellular amino acids, microdialysis was conducted in spinal cord during and after administration of impact trauma. Extracellular concentrations of excitatory, inhibitory, and nontransmitter amino acids were elevated after trauma, with the degree of increase related to severity of injury. Moderate trauma resulted in an immediate but transient increase (200-400%) in the extracellular levels of all amino acids measured. Severe trauma produced a more prolonged and significant increase (400-630%) in the concentrations of extracellular amino acids, including aspartate and glutamate. These results are consistent with the hypothesis that excitatory amino acids may contribute to delayed tissue injury after central nervous system trauma.