The neural substrates of anxiety and the suppression of anxiety will be studied in humans using functional neuroimaging techniques. Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) scans will be performed in healthy subjects who have been made anxious by behavioral manipulations such as the threat of painful electric shock. The goal of this research is to identify specific limbic and prefrontal brain regions that are involved in the generation, maintenance, and extinction of anxious emotional states. This research has implications for a variety of anxiety disorders such as panic disorder, simple phobias, and post-traumatic stress disorder.