We used [18F]cyclofoxy (CF), a potent opiate antagonist with affinity for mu and kappa receptors, and the Scanditronix PC1024-7B PET scanner to study 14 patients with complex partial seizures (CPS), and 14 normal controls. Epileptic foci were localized by prolonged EEG-video monitoring. EEG was recorded continuously during each scan. Immediately before CF administration, [15O]labeled water was used to measure cerebral blood flow, and showed hypoperfusion ipsilateral to the EEG focus. Blood samples (corrected for radiolabeled metabolites) and tissue time-activity data were acquired over 90 min following bolus CF injection. Anatomic regions were outlined directly on the PET images. A kinetic model was used to derive the total volume of distribution (Vt) in each brain region. Specific binding (Vs) was determined by substracting non-specific binding (Vt) measured in a receptor-poor brain region (occipital cortex). Regions with high Vs included mesial temporal lobes, thalamus, basal ganglia, and frontal cortex. Individual patients appeared to have higher binding in temporal lobe ipsilateral to the EEG focus, but there was no asymmetry for the patients as a group in mean Vt or Vs in anterior mesial, posterior mesial, anterior lateral, posterior lateral temporal cortex, thalamus, basal ganglia, or, for Vt, in regions of low specific binding: occipital lobe, parietal lobe, cerebellum.