Amino acids are of crucial importance in brain function, not only as metabolic intermediates and building blocks of proteins, but also as mediators of interneuronal communication. This dual role of the amino acids distinguishes them from other neurotransmitter candidates, and implies that they are unlikely to be restricted to neurons using them as transmitters. This calls for a quantitative approach when attempts are made to analyse the distribution of transmitter amino acids by means of immunocytochemistry. The present review deals with recent methodological developments that have made it possible to utilize specific antisera to explore the cellular and subcellular distribution of neuroactive amino acids in a quantitative manner.