The M1-selective antiulcer drug pirenzepine (1) is a tricyclic compound with close resemblance to tricyclic psychotropic agents such as imipramine (2). Despite this fact, pirenzepine is devoid of any psychotropic effects, exhibiting measurable antagonistic effects in biochemical assays and receptor binding studies only toward the muscarinic receptor system. To understand how different groups in these tricyclic molecules affect binding affinities, a set of nine compounds structurally related to pirenzepine (1) and imipramine (2) has been selected for analysis, comprising three different tricycles and three different side chains. The compounds were tested for their affinity to the imipramine and muscarinic receptors in homogenized rat cortex tissue. The result of these studies suggests that it is the nature and placement of accessory groups that determine the differences in receptor recognition and the binding process. In the case of pirenzepine (1), preferential binding toward the muscarinic receptor is brought about by the endocyclic amide group, by the positioning of the protonated N atom of the side chain, and to a minor extent by the exocyclic amide group. From these findings a putative model for the explanation of selective binding of pirenzepine (1) to the muscarinic receptor has been derived.