 Evidence suggests that the expectation of pain relief, even if a person receives only a placebo, can provide actual therapeutic benefits. The μ-opioid receptor system, located in the brain, is activated during anticipation of pain relief; this activation suppresses stress and pain responses. This study will use brain imaging technology to examine the effects of a placebo intervention on μ-opioid neurotransmitters. Examination of the factors that regulate these placebo-activated neurotransmitter responses will clarify the overall neurobiology underlying variations in the responses to placebos, as well as pain and other stressful conditions, ultimately leading to the optimization of medical and psychological interventions. 