1. The effect of amitriptyline on human serum butyrylcholinesterase (acylcholine acylhydrolase E.C.3.1.1.8) has been investigated. From the Lineweaver-Burk plot and the plot of v versus amitriptyline concentration, it was concluded that amitriptyline inhibition is partially competitive, and the kinetic parameters have been calculated as Ks = 0.11 mM, alpha = 1425 and Ki = 0.01 mM. 2. Because amitriptyline is a partial competitive inhibitor of butyrylcholinesterase, acquired deficiency may be seen in patients treated with amitriptyline and may cause complications in operations.