2-[C]Methyl-6-(2-phenylethynyl)pyridine ([C]MPEP) is a radioligand developed for positron emission tomography (PET) imaging of metabotropic glutamate (mGlu) receptor subtype 5 (mGluR5 or mGluR) in the central nervous system (CNS) (1). C is a positron emitter with a physical half-life () of 20.3 min. Glutamate is a major excitatory neurotransmitter at CNS synapses. Many neuroanatomical CNS projection pathways contain glutamatergic neurons (2). Glutamate produces its excitatory effects by acting on cell-surface ionotropic glutamate or mGluRs (3). The mGluRs are G-protein–coupled receptors, and the eight mGluR subtypes are further subdivided into groups I, II, and III. The group I receptors include mGluR1 and mGluR5, and they are found predominantly in postsynaptic locations. The mGluR5s are found with high to moderate density in the frontal cortex, caudate, putamen, nucleus accumbens, olfactory tubercle, hippocampus, and dorsal horn of the spinal cord, whereas the density in the cerebellum is low. These receptors are coupled to phospholipase C and up- or downregulate neuronal excitability. They have been implicated in a variety of diseases in the CNS, including anxiety, depression, schizophrenia, Parkinson’s disease, and drug addiction or withdrawal. These receptors are also involved in the modulation of various pain states, which makes them attractive targets for therapeutic drug development. PET and single-photon emission computed tomography imaging of radioligands that target mGluR5s can be used to visualize and study the CNS mGluR5s in normal and pathological states. Some mGluR5 antagonists have been successfully labeled, but their visualization has been hampered by high lipophilicity, unfavorable brain uptake kinetics, or high metabolism (4, 5). MPEP and its methyl analog M-MPEP have been identified as potent and highly selective noncompetitive antagonists for mGlu5 (1, 6). Yu et al. (1) synthesized [C]MPEP and demonstrated the feasibility of using it as a PET imaging ligand for imaging.