An important aspect of brain cholinesterase function is related to enzymatic differences. The brain of mammals contains two major forms of cholinesterases: acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). The two forms differ genetically, structurally and for their kinetics. Butyrylcholine is not a physiological substrate in mammalian brain which makes the function of BuChE of difficult interpretation. In human brain, BuChE is found in neurons and glial cells as well as in neuritic plaques and tangles in Alzheimer disease (AD) patients. While AChE activity decreases progressively in the brain of AD patients, BuChE activity shows some increase. In order to study the function of BuChE, we perfused intracortically the rat brain with a selective BuChE inhibitor and found that extracellular acetylcholine increased 15 fold from 5 to 75nM concentrations with little cholinergic side effects in the animal. Based on these data and on clinical data showing a relation between CSF BuChE inhibition and cognitive function in AD patients, we postulated that two pools of cholinesterases may be present in brain, the first mainly neuronal and AChE dependent and the second mainly glial and BuChE dependent. The two pools show different kinetic properties with regard to regulation of ACh concentration in brain and can be separated with selective inhibitors. Within particular conditions, such as in mice nullizygote for AChE or in AD patients at advanced stages of the disease, BuChE may replace AChE in hydrolyzing brain acetylcholine. Based on the changes of ChE activity in the brain of AD patients, a rational indication of selective BuChEI (or of mixed double function inhibitors) is the treatment of advanced cases. A second novel aspect of ChEI therapy is the emerging of new indications which include various forms of dementia such as dementia with Lewy Bodies, Down Syndrome, vascular dementia and Parkinson Dementia. Clinical results demonstrate examples of versatility of cholinergic enhancement.