One of the major problems in glutamate immunocytochemistry has been the difficulty involved in separating immunocytochemical labelling due to metabolic glutamate from the labelling caused by transmitter glutamate. Another problem appears to be the accessibility of antigenic sites in conventional light microscopic preparations. In the present report, we have applied the primary glutamate antiserum onto ultrathin tissue sections, followed by the use of a colloidal gold detection system. The use of this postembedding immunogold procedure allows equal access of antibodies to all cellular compartments exposed at the section surface, allows quantitative assessment of the immunoreactivity, and affords a high resolution compatible with studies at the organelle level. When applied to slice preparations the immunogold procedure can be used to identify releasable pools of glutamate. These methodological advances have greatly increased the usefulness of glutamate immunocytochemistry as a tool to study putative glutamatergic terminals in the CNS.