Glutamate levels were determined in the fasting plasma of 22 patients with early-stage primary amyotrophic lateral sclerosis (ALS) and compared to those of healthy and diseased controls. There was a significant increase (by approximately 100%, p less than 0.0005) in the plasma glutamate of the ALS patients as compared with the controls. Oral glutamate loading (60 mg of monosodium glutamate per kilogram of body weight, taken orally after overnight fasting) resulted in significantly greater elevations in the plasma glutamate and aspartate levels in the ALS patients than in the controls. Glutamate, a potentially neuroexcitotoxic compound, is thought to be the transmitter of the corticospinal tracts and certain spinal cord interneurons. A systemic defect in the metabolism of this amino acid may underlie primary ALS.