Acetylcholine (ACh) is one of the most important transmitters in the nervous system. Presently, it is assumed that the following sequence of events occurs in a cholinergic synapse: ACh (1) is relased by a nerve cell, (2) binds to receptor proteins on a neighbouring cell (e.g. muscle, nerve, gland, etc.) and (3) is rapidly degraded by acetylcholinesterase (AChE). This theory rests on the fundamental postulate that hydrolysis of ACh by AChE leads to its deactivation (in the sense of reducing or eliminating ACh’s excitatory effect).This widespread assumption was criticised by Kaufmann (Kaufmann, 1977) and is indeed falsified herein. It is demonstrated that excitable plant cells (Chara australis internodes), which had previously been unresponsive to ACh, were rendered ACh-sensitive in the presence of AChE. This was underlined by a striking difference in cell membrane depolarisation upon exposure to intact ACh (ΔV=-2±5 mV average±StDev (n=6)) and its hydrolysate respectively (ΔV=81±19 mV (n=14)). This pronounced depolarisation, which also triggered action potentials, was clearly attributed to one of the hydrolysis products: acetic acid (ΔV=87±9 mV at pH 4.0 (n=5); choline ineffective in range 1-10 mM (n=3)).Our findings suggest that AChE does not clean up the synapse, but actually generates an excitatory agent (acetic acid). Protons excite the plant cells used in this study as well as a large variety of other systems including gels (Walters et al., 1961), lipid membranes, erythrocytes (Holland and Graham, 1955), oocytes (Burckhardt et al., 1992) and neurons (Krishtal and Pidoplichko, 1980). Whether excitation of the postsynaptic cell is due to protons and/or due to intact ACh, is a fundamental question that has not received any attention up to date.