1. The L-2-amino-4-phosphonobutyric acid (L-AP4) receptor was originally discovered by the ability of L-AP4 to depress synaptic transmission in hippocampal glutamatergic pathways and in the retina. 2. The molecular identity of the L-AP4 receptor is not yet resolved; however, with the molecular cloning of subtypes of metabotropic glutamate receptors (mGluRs), high affinity targets for L-AP4 have been identified. 3. As the information on the pharmacology of the mGluRs and the electrophysiological and biochemical studies on L-AP4 receptor physiology becomes elaborated it seems evident that the L-AP4 receptor is not a single molecular target but may involve multiple receptor subtypes.