Depositions of amyloid plaques, neurofibrillary tangles and subsequent neurotransmitter deficits are neuropathological characteristics of Alzheimer's disease. Non-invasive monitoring of these changes in the brain is potentially useful for early diagnosis of Alzheimer's disease. Here we introduce the method for measuring the density of amyloid plaques and neurotransmitters using 11C-labeled donepezil, doxepin and BF-227. Positron emission tomography (PET) imaging by [11C]donepezil visualized the distribution of acetylcholine esterase in the brain. PET study using [11C]doxepin also demonstrated the distribution of histamine H1 receptor in the brain. Density of acetylcholine esterase and H1 receptor was prominently reduced in the patients with Alzheimer's disease. [11C]BF-227 PET successfully visualized neocortical depositions of amyloid plaques in the brain and clearly differentiated the patients with Alzheimer's disease from healthy individuals. These tracers would be useful for early and accurate diagnosis of dementia and monitoring anti-dementia therapy.