Three female patients aged from 74 to 79 with multi-infarct dementia were studied using positron emission tomography (PET) to assess the effect of co-dergocrine mesylate (Hydergine) on cerebral glucose metabolism. The cerebral glucose utilization (CMRGlc) of each patient was evaluated by PET scan using 2-deoxy-[18F]-2-fluoro-D-glucose (FDG). Following the first PET study, 0.04 mg/kg of co-dergocrine mesylate was injected intravenously with 250 ml saline solution, and then the second PET study was performed. The CMRGlc was determined from the images of the PET scan and the radioactivity of 18F in the plasma. After the administration of co-dergocrine mesylate, the value of CMRGlc increased significantly in the cerebral cortex (p less than 0.01 and p less than 0.05) and basal ganglia (p less than 0.05) compared with values before the administration, but no significant increase was found in the centrum semiovale. These results suggest that co-dergocrine mesylate stimulates glucose metabolism of neurons in the human brain.