Regional cerebral metabolic rates for glucose (rCMRglc) were studied in unanesthetized Beagle dogs in five age groups. Significant age-related differences did not occur in the cingulate, pyriform or visual cortices, cerebellar flocculus, corpus callosum, or cerebellar white matter. However, age-related decrements were apparent in 15 of the 22 brain regions examined. The apparent time course of age effect on rCMRglc varied among the brain regions. Most regions had significantly lower rCMRglc at 6 years than at 1 year. Decrements of more than 25% were seen in the mammillary bodies, pons, hippocampus, superior colliculus, basis of the midbrain, temporal cortex, geniculate bodies, caudate nucleus, and superior frontal gyrus. There were no age differences in rCMRglc at 10-12 years compared with 6 years. Senescence-associated decrements (after 6 years) were noted in only 5 regions: the frontal and temporal cortices, mammillary bodies, and areas involved in sensory functions. The results indicate that rCMRglc in the adult Beagle brain declines by midlife, and continues to decline in some brain regions through senescence.