Several studies of human brain postmortem report that monoamine oxidase B (MAO B) increases with age and it has been proposed that this increase reflects age-associated increases in glial cells. We measured brain MAO B in a group of normal healthy human subjects (n = 21; age range 23-86; 9 females and 12 males; nonsmokers) using [11C]L-deprenyl-D2 and positron emission tomography. Brain glucose metabolism was also measured with 18FDG in 15 of the subjects. MAO B increased (p < 0.004) in all brain regions examined except the cingulate gyrus. In contrast, subjects showed the expected regional age-related decreases in blood flow and metabolism. In the 15 subjects in whom both MAO B and LCMRglu was measured, there was a trend (p < 0.03) toward an inverse association between brain glucose metabolism and MAO B activity in the frontal and parietal cortices. Although the age-related increase in brain MAO B in living subjects is consistent with postmortem reports, the degree of increase is generally lower.