1. The inhibition of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) by compounds containing trifluoromethyl-carbonyl groups was investigated and related to the effects observed with structurally similar, non-fluorinated chemicals. 2. Compounds that in aqueous solution readily form hydrates inhibit acetylcholinesterase in a time-dependent process. On the other hand non-hydrated, carbonyl-containing compounds showed rapid and reversible, time-independent enzyme inactivation when assayed under steady state conditions. 3. m-N,N,N-Trimethylammonium-acetophenone acts as a rapid and reversible, time-independent, linear competitive inhibitor of acetylcholinesterase (Ki = 5.0 . 10(-7) M). 4. The most potent enzyme inhibitor tested in this series was N,N,N,-trimethylammonium-m-trifluoroacetophenone. It gives time-dependent inhibition and the concentration which inactivates eel acetylcholinesterase to 50% of the original activity after 30 min exposure is 1.3 . 10(-8) M. The bimolecular rate constant for this reaction is 1.8 . 10(6) 1 . mol-1 . min-1. The enzyme-inhibitor complex is very stable as the inhibited enzyme after 8 days of dialysis is reactivated to 20% only. This compound represents a quasi-substrate inhibitor of acetylcholinesterase.