Regional cerebral blood flow (rCBF) and regional cerebral metabolic rate of glucose (rCMRglc) were measured in aged and young monkeys by positron emission tomography (PET). Our purpose was to examine whether the age-related changes observed in the human brain also occur in the monkey brain. Studies were performed on six aged and six young-adult male rhesus monkeys (Macaca mulatta). rCBF and the rCMRglc were serially measured using PET with [(15)O]H(2)O and 2-[(18)F]fluoro-2-deoxy-D-glucose (FDG), respectively. In order to minimize the bias induced by anesthesia, the PET emission scans were performed in the conscious state. ROIs were taken for the cerebellum, hippocampus with adjacent cortex, striatum, occipital cortex, temporal cortex, frontal cortex and cingulate. Group differences and correlations between rCBF and rCMRglc in each group were determined. Aged monkeys had significantly lower rCBF in the cerebellum, hippocampus with the adjacent cortex, striatum, occipital cortex, temporal cortex, frontal cortex, and significantly lower rCMRglc in the cerebellum, hippocampus with the adjacent cortex, striatum, occipital cortex, temporal cortex, frontal cortex and cingulate, compared to young monkeys. There were significant correlations between rCBF and rCMRglc in both the aged and young groups, but no significant difference was found in relationship between the two groups. Age-related changes were observed not only in rCMRglc, but also in rCBF in aged monkeys, while the coupling between rCBF and rCMRglc was maintained even in aged monkeys. These results demonstrated the potential of aged monkeys to serve as an aged human model using PET.