on the athletes at two separate times: at rest, and after a two-hour bout of endurance running. In the PET, they used a radioactive opioidergic ligand 6-O-(2-[F] fluoroethyl)-6-O-desmethyldiprenorphine ([18F]FDPN). This substance binds to opiate receptors in the brain. If opiate receptors are in use by endorphins, the [18F]FDPN should be unable