Glutamate, aspartate and possibly other excitatory acidic amino acids are thought to be, neurotransmitters at the majority of excitatory synapses in the vertebrate CSN. The synaptic response elicited by excitatory amino acids is mediated by at least four (probably five) different receptor subtypes. These were known as the N-methyl-D-aspartate, quisqualate, kainate and L-AP4 receptor subtypes. More recently a fifth receptor subtype has been discovered that is linked to phosphoinositol metabolism. The most well characterized excitatory amino acid receptor subtype is the NMDA receptor. This receptor consists of a recognition site for NMDA, a cation-selective ion channel and binding site for glycine, Zn2+ and phencyclidine-like compounds. In addition the channel can be blocked by Mg2+. New and selective ligands and radioligands have facilitated mapping the distribution of the major excitatory receptor subtypes in normal and diseased brain, examines allosteric interactions within the NMDA receptor, searching for novel therapeutic agents and determining drugs mechanisms.