The hydrolysis of acetylthiocholine by acetylcholinesterase fromElectro-phorus electricuswas investigated in the presence of the inhibitors tacrine, gallamine and compound1. The interaction of the enzyme with the sub-strate and the inhibitors was characterized by the parametersKI , a¢, borb, Kmand Vmax, which were determined directly and simultaneously from nonlinear Michaelis–Menten plots.