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Much fundamental information about the structure of the active centre in cholinesterases has been derived from studies on changes of activity with pH. These enzymes show a maximum activity at about pH 7-8. On the acid side, activity falls off in approximately the same manner for all substrates studied. However, in the alkaline range great differences were observed between different esters (Bergmann, Segal, Shimoni & Wurzel, 1956). The bell-shaped form of the pH-activity curves has been interpreted to indicate two different components of the esteratic site, one of which (the nucleophilic group G1) is inactivated competitively