Alzheimer's disease (AD) and Pick's disease (PD) are dementing disorders of unknown origin. Monoamine oxidase (MAO) is important in the metabolism of a number of monoamine neurotransmitters. This study investigates MAO activity in aging, AD, and PD in frontal and temporal poles, nucleus basalis of Meynert (nbM), and hypothalamus. Regional variations in the activity of the two MAO forms (A and B) were found in aging, AD, and PD. Age-related increase of MAO-B was found in all four areas of the brain, and MAO-A was increased in the nbM and temporal pole. In AD, MAO-B was decreased in the nbM and increased in the temporal pole, while MAO-A was increased in the hypothalamus and frontal pole. In PD, MAO-B was decreased in the nbM and increased in the hypothalamus, while MAO-A was increased in the hypothalamus and decreased in the nbM and temporal pole. These data indicate that with regard to MAO activity, AD and PD are dissimilar diseases and neither strictly follows age-related changes.