The purpose of this protocol is to advance the understanding of the behavioral neurobiology underlying substance dependency by exploring regional brain activity in response to appetitive and aversive stimuli in human subjects. In this protocol, brain activation is the dependent measure, as measured by blood oxygenation level dependent (BOLD) signal acquired during functional magnetic resonance (fMRI) imaging. Previous work in this laboratory has found that cues of anticipated monetary reward versus threat of negative reinforcement causes increased BOLD signal in the nucleus accumbens (NAcc) and other parts of the ventral striatum. This protocol is intended to extend these findings by exploring whether the recently reported BOLD activation (a hemodynamic response) in certain brain regions in response to motivational cues can be perturbed by dopaminergic pharmacological manipulations.