The application of molecular cloning technology to the study of the glutamate receptor system has led to an explosion of knowledge about the structure, expression, and function of this most important fast excitatory transmitter system in the mammalian brain. The first functional ionotropic glutamate receptor was cloned in 1989 (Hollmann et al 1989), and the results of this molecular-based approach over the past three years are the focus of this review. We discuss the implications of and the new questions raised by this work-which is probably only a glance at this fascinating and complex signaling system found in brains from the snails to man. Glutamate receptors are found throughout the mammalian brain, where they constitute the major excitatory transmitter system. The longest-known and best-studied glutamate receptors are ligand-gated ion channels, also called ionotropic glutamate receptors, which are permeable to cations. They have traditionally been classified into three broad subtypes based upon pharmaco logical and electrophysiological data: a-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors, kainate (KA) receptors, and N-methyl-D-aspartate (NMDA) receptors. Recently, however, a family of G protein-coupled glutamate receptors, which are also called metabotropic glutamate or trans-l-aminocyclopentane-l, 3-dicarboxylate (tACPD) recep tors, was identified (Sugiyama et al 1987).(For reviews of the classification and the pharmacological and electrophysiological properties of glutamate receptors see Mayer & Westbrook 1987, Collingridge & Lester 1989, Honore 1989, Monaghan et al 1989, Wroblewski & Danysz 1989, Hansen &