Steroidal thiazolium salts were prepared and their interaction with [3H]N-methylscopolamine ([3H]NMS) at M2 muscarinic receptors using radioligand binding were studied. The results were consistent with a ternary complex theory. The compounds diminished [3H]NMS binding with Kd values in the micromolar or submicromolar range. In particular, the presence of bulky benzylic group at the thiazole nitrogen atom caused a substantial increase in affinity of allosteric ligand for receptor. Our results demonstrate that steroidal thiazolium salts could be a new class of potentially useful tools for mapping the muscarinic receptors.