Acetylcholineasterase (AChE) is present in sensory innervated tissues and it functions in the central and peripheral neuron system to terminate nerve signal transduction at the neuromuscular junction by rapid hydrolysis of the acetylcholine (ACh) receptor released into the cholinergic synaptic cleft. AChE is also found in human red blood cells (RBCs) (Brauer and Root, 1947), and it is one of the typical extrinsic membrane bound enzymes (Heller and Hanahan, 1972). Although the physiological functions of AChE in RBC remain obscure, changes in activity associated with pathologic conditions are found regularly only with AChE. Recent studies have disclosed much of the primary structure of AChE and the membrane anchor structure. However, the location of this enzyme at or near the cell surface gives it special significance in studies of cellular membranes and the activity alterations seen in several hemolytic disorders may be of importance in understanding certain basic disease process. The enzyme may be regarded as a model of AChE in the nervous system. AChE inhibition has been used as a peripheral surrogate biomarker for the activity of centrally acting AChE inhibitors (AChEIs) in the treatment of Alzheimer's disease (AD). AChE inhibition in RBC should reflect the central pharmacodynamic activity of the compound and the degree of inhibition should correlate to yielding maximum cognitive or global improvement in patients with AD. AChEI is also a useful clinical tool in dose optimization!