4-(M-Chlorophenylcarbamoyloxy)-2-butynyl)trimethylammonium (McN-A-343) is a selective muscarinic M1 receptors agonist, which leads to its widespread use as an aid to distinguish responses mediated through M₁ receptors from those utilizing M₂ or M₃ muscarinic receptor subtypes, especially in the CNS. McN-A-343 has a number of non-muscarinic actions. These include activation of some types of nicotinic acetylcholine receptors, antagonism of serotonin 5-HT₃ and 5-HT₄ receptor subtypes, inhibition of the uptake mechanism and a local anesthetic action.