It is generally thought that up to 90% of all synapses in the mammalian brain use amino acids as neurotransmitters (1 ). Neurotransmitter amino acids may either be those that are also used directly as protein constituents (such as glutamate) or modified amino acids, such as γ-amino butyric acid (GABA). In addition, there are other amino acid-derived transmitters, such as the tryptophan derivative 5-hydroxytryptamine (serotonin), the tyrosine-derived transmitters, such as dopamine, and the histidine derivative, histamine. Although these latter groups are not normally considered to be amino acid neurotransmitters per se (because of the removal of the acidic carboxyl group), the methodologies outlined in this chapter are still directly applicable to their immunolocalization.