Imaging studies using symptom-provocation paradigms in specific phobia have yielded contradictory results, possibly reflecting a failure to account for habituation processes. Given that a single session of exposure in vivo can result in significant improvement in specific phobia, we used prolonged exposure to phobic stimuli to identify CNS regions showing habituation. Eighteen subjects (12 with spider phobia, 6 healthy controls) underwent H(2)(15)O-positron emission tomography while being continuously presented with pictures of spiders or butterflies. Results showed main effects (spiders>butterflies) in the phobia group in the left fusiform gyrus (FG) and the right parahippocampal gyrus (PHG), with bilateral perirhinal cortex and right limbic areas approaching significance. Group x condition effects were found in the right amygdala and PHG. During spider scans, large habituation effects were observed in the anterior medial temporal lobe (MTL) bilaterally. Regression analyses demonstrated that state anxiety was correlated with activity in left amygdala, bilateral perirhinal cortex, right FG, and periaquaductal grey; by contrast, phobic fear was only associated with right-sided hippocampal activity. We conclude that prolonged exposure to phobic stimuli is associated with a significant decrease in bilateral anterior MTL regional cerebral blood flow. Right anterior MTL, identified when comparing phobic vs. neutral stimuli and habituation to phobic vs. neutral stimuli in the phobia group, implicates this region in phobic fear. Analyses of covariance suggest a further functional segregation with state anxiety being linked to enhanced activity in amygdala, perirhinal cortex, and tegmentum, and phobic fear with enhanced right hippocampal activity, suggesting a neuroanatomical differentiation between emotional-vegetative and cognitive aspects of (phobic) fear.