The glutamate content of synaptosomal-enriched fractions is stable during the preparation and isolation of the organelles provided that the temperature of the preparation is maintained at 0--2 degrees C and that inorganic ions are not added to the homogenizing and centrifugation media. Consequently, drug-induced in vivo changes in the glutamate content of nerve endings should be reflected by the corresponding changes in glutamate levels of synaptosomes prepared from the brain tissue.