The effect of several antemortem and postmortem factors (patients' age, sex, postmortem delay, storage time, laterality and brain weight) on both monoamine oxidase-A (MAO-A) and -B (MAO-B) activity was investigated in the frontal cortex of human brains. The MAO-A activity decreases rapidly during the first two years of life and remains constant thereafter. In contrast, the MAO-B activity keeps unchanged during early childhood and raises during advanced age. These findings seem to be consistent with a genetic regulation and a variation in cell type assembling during brain development and aging.