Current approaches to the treatment of cognitive and behavioral symptoms of Alzheimer disease emphasize the use of cholinesterase inhibitors. The kinetic effects of the cholinesterase inhibitors donepezil, galantamine, metrifonate, physostigmine, rivastigmine, and tetrahydroaminoacridine were examined with respect to their action on the esterase and aryl acylamidase activities of human acetylcholinesterase (AChE) and human butyrylcholinesterase (BuChE). Each of these drugs inhibited both AChE and BuChE, but to different degrees. Inhibition of BuChE by these compounds was approximately the same, or better, when acetylthiocholine, the analog of the neurotransmitter acetylcholine, was used as the substrate, instead of butyrylthiocholine. In addition, for these drugs, the inhibition of aryl acylamidase activity paralleled that observed for inhibition of esterase activity of AChE and BuChE. Given that drugs that are currently in use for the treatment of Alzheimer disease inhibit both AChE and BuChE, the development of drugs targeted toward the exclusive inhibition of one or the other cholinesterase may be important for understanding the relative importance of inhibition of BuChE and AChE in the treatment of this disease.