Regional cerebral blood flow (rCBF) was measured in 35 brain regions of awake beagles of different ages, by infusing 14C-iodoantipyrine intravenously. Between 1-3 and 6-12 yrs of age, rCBF was reduced significantly (P less than 0.01) in only 6 of the 35 regions, by 16 to 25%, and mean weighted CBF was not significantly altered. At 14-15 yrs, rCBF was reduced significantly by 25 to 50% in 13 of 28 grey matter regions examined, but in no white matter region, as compared with rCBF at 1-3 yrs of age, and mean weighted CBF was reduced by 30% (P less than 0.01). The course of rCBF during ageing of the beagle corresponded to the reported course of the regional cerebral metabolic rate for glucose. The results demonstrate that cerebral functional activity in most brain regions is constant during much of the adult life of the beagle, and that coupling between blood flow and metabolism also is constant. Compensatory morphological and neurochemical mechanisms may account for this homeostasis of CBF. In extreme senescence, generalized reductions in rCBF are found, and probably reflect reduced sensory input to the brain as well as systemic disease.