Immunocytochemical methods for peptides and serotonin have greatly advanced the study of neurones in which these substances are likely to be transmitters. Such direct techniques have not so far been available for the amino acid transmitter candidates. We report here the selective immunocytochemical visualization of the putative transmitters glutamate (Glu) and gamma-aminobutyrate (GABA) by the use of antibodies raised against the amino acids coupled to bovine serum albumin (BSA) with glutaraldehyde (GA). The tissue localizations of Glu-like and GABA-like immunoreactivities (Glu-LI and GABA-LI) matched those of specific uptake sites for Glu and GABA, and, in the case of GABA-LI, also that of the specific marker enzyme glutamic acid decarboxylase (GAD). Thus, GABA-LI was located in what are believed to be GABAergic inhibitory neurones, whereas Glu-LI was concentrated in excitatory, possibly glutamatergic neurones. Preliminary electron microscopic observations suggest that the transmitter amino acids are significantly concentrated in synaptic vesicles.