ABP 688 is a non-competitive antagonist of metabotropic glutamate receptor 5 (mGluR5; K = 3.5 nM in a radioligand binding assay using L(tk-) cell membranes expressing human receptors). It is selective for mGluR5 receptors over a panel of CNS G protein-coupled receptors, ion channels, and transporters at 10 μM. ABP 688 inhibits quisqualate-induced phosphoinositol accumulation in and glutamate-induced calcium release from L(tk-) cells expressing human mGluR5 (ICs = 2.4 and 2.3 nM, respectively). Radiolabeled forms of ABP 688 have been used as PET tracers for in vivo imaging of mGluR5 receptors in rodents.