SUMMARY The oxygen-15 continuous inhalation technique and PET were used to study the age-related changes in regional CBF and CMRO 2 . Twenty-seven patients, aged 19 to 76 years, free of any history of cerebral disease and vascular risk factors were examined in "resting state." CBF, CMRO 2 and oxygen extraction fraction (OEF) values were calculated in seven different brain structures as well as in mean gray matter. Left-right ratios were also computed for all symmetrical structures analyzed.Mean gray CBF, but not mean gray CMRO 2 , decreased linearly with age (p < 0.02). However, when younger subjects (S50 yrs) were compared to older subjects (>50 yrs), an age-related matched decrease in CBF and CMRO 2 was observed in mean gray matter (18% and 17%, p < 0.05) and in all gray matter regions analyzed, particularly in frontal, temporo-sylvian and parieto-occipital cortex. White matter CBF and CMRO 2 remained remarkably stable with advancing age.Although the possibility'of methodological artifacts was considered, we favor progressive loss of cortical neurones and/or diminished activity of those remaining to explain our findings. In addition, age-related changes in cognitive activities might also be involved. Stroke Vol 15, No 4, 1984 DESPITE NUMEROUS STUDIES, 114 the effects of aging on cerebral circulation and metabolism still remain largely unsettled. Studies on global hemispheric cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO 2 ) have so far provided discrepant results: some^ concluded that CBF and CMRO 2 did not decrease with advancing age except when vascular risk factors were present, while other authors 1 2 found a parallel decline in both CBF and CMRO 2 . These discrepancies may be partially explained by different criteria for normality used. In his review on this subject, Kety 7 favored the idea that there was a rapid fall of both CBF and CMRO 2 around puberty which continued to the third decade and was followed by a more gradual decline in middle and old age.

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