In a series of two, repeated-measures, within-subject experiments, we propose to temporarily alter subjects' brain monoamine neurotransmitter systems with pharmacological interventions prior to presenting the subjects with appetitive and aversive visual stimuli as well as cues signaling potential punishment and reward. In the first experiment, brain catecholamine systems will be deactivated by dietary restrictions of amino acid precursors Tyrosine (Tyr) and Phenylalanine (Phe), respectively. In the second experiment, the brain dopamine (DA) release will be transiently increased with administration of dextroamphetamine (Amph). Based on a wealth of existent findings, we predict that acutely enhancing synaptic DA by Amph will augment reward-elicited activation in the ventral striatum. Conversely, acute dietary restriction of the DA precursor Tyr, (along with Tyr precursor Phe) will blunt this activation. Since substance use disorders (SUD) such as alcoholism may result from disordered neural systems regulating approach and avoidance behavior (with respect to the substance), this experiment can yield useful knowledge relevant to SUD.