Acetylcholine is the neurotransmitter of the parasympathetic system and is stored in small oval vesicles in the cholinergic nerve terminals. Acetylcholine is released into the synaptic cleft where it is rapidly destroyed by the enzyme acetylcholinesterase. Acetylcholine acts on muscarinic and nicotinic receptors. Cholinergic drugs (parasympathomimetics) are chemicals that act at the same sites as acetylcholine and mimic its actions. Esters of choline are effective orally. Pilocarpine, a cholinomimetic alkaloid, has prominent muscarinic actions and is used in glaucoma. Anticholinesterases inhibit the enzyme cholinesterase, resulting in accumulation of acetylcholine and their actions are due to accumulated acetylcholine. Anticholinesterases may be reversible or irreversible. Physostigmine is used in glaucoma and atropine poisoning. Reversible anticholinesterases are also used in myasthenia gravis, poisoning due to anticholinergic drugs, curare poisoning, post operative paralytic ileus and urinary retention and in cobra bite. Rivastigmine and donepezil are tried in Alzheimer’s disease. Organophosphorus poisoning is quite common and symptoms result from muscarinic, nicotinic and central effects. Atropine and cholinesterase reactivators are used in the treatment of organophosphorus poisoning.