3-Quinuclidinyl thiochromane-4-carboxylate is a research compound which is the most potent muscarinic antagonist known. Tests in vitro showed it to have a binding affinity over 2100 times more potent than 3-quinuclidinyl benzilate (BZ) with an apparent association constant (K<sub>A</sub>) of 2.47 picomolar (pM).