Positron emission tomographic measurements of regional blood flow were used to assess local neuronal activity in patients with panic disorder and in normal control subjects before and during the infusion of sodium lactate. A new technique for the analysis of positron emission tomographic data was employed to identify significant changes in regional blood flow associated with lactate infusion in the panicking patients, nonpanicking patients, and controls. Lactate-induced panic was associated with significant blood flow increases bilaterally in the temporal poles; bilaterally in insular cortex, claustrum, or lateral putamen; bilaterally in or near the superior colliculus; and in or near the left anterior cerebellar vermis. Lactate infusion was not associated with significant changes in regional blood flow in the nonpanicking patients or control subjects. Thus, the identified regions seemed to be involved in an anxiety attack.