To investigate the role of glutamic (Glu) and aspartic acid (Asp) in migraine, we measured the plasma amino acids in migraine patients with and without aura, between and during attacks, and compared the profiles with the plasma amino acid profiles of tension headache patients and healthy controls. Between attacks, migraineurs (notably with aura) had substantially higher plasma Glu and Asp levels than did controls and tension headache patients. In addition, patients with migraine without aura showed low plasma histidine levels. During migraine attacks, Glu (and to a lesser extent Asp) levels were even further increased. The results suggest a defective cellular reuptake mechanism for Glu and Asp in migraineurs, and we hypothesize a similar defect at the neuronal/glial cell level, predisposing the brain of migraineurs to develop spreading depression.