MePPEP is a ligand that is highly selective for the cannabinoid type 1 (CB1) receptor. This receptor is considered the most common G-coupled protein receptor in the brain (Pacher et al., 2006). The CB1 receptor is the site of action of Delta-9-tetrahydrocannbinol (THC), the active compound in marijuana, and is found primarily on the presynaptic terminals of dopaminergic, glutamatergic, GABAergic neurons (Howlett et al., 2002). The function of the CB1 receptor is not entirely clear; however, it has been implicated in several neurological and psychiatric disorders, and a selective inverse agonist, rimonabant, is currently in use in Europe for the treatment of obesity (Van Gaal et al., 2005). [11C]MePPEP was developed to enhance our understanding of the in vivo characteristics of the CB1 receptor (e.g., receptor density and receptor occupancy with pharmaceuticals).