The chemical reactions and pathways involving acetylcholine, the acetic acid ester of the organic base choline. Acetylcholine is a major neurotransmitter and neuromodulator both in the central and peripheral nervous systems. It also acts as a paracrine signal in various non-neural tissues. [ GOC:jl GOC:nln https://www.worldcat.org/search?q=bn%3A0192800752 ]