The purpose of this study is to determine whether DAT availability, assessed by WIN binding, in the striatum is altered in cocaine or methamphetamine dependence. To determine whether DA synthesis capacity, assessed by FDOPA uptake, in the striatum is altered in Coc or Meth dependence. To determine whether the PET tracers, WIN or FDOPA, will differentiate Meth induced alterations from those induced by Coc use. To determine whether the PET characterization of striatal alterations observed at 3-5 days since last drug use persists at least 3 months after last drug use.