In order to gain some insight into those structural features that control M1 selectivity, a selected set of pirenzepine analogues has been studied in which both the tricyclic ring system and the basic side chain have been varied. Binding studies were conducted in rat tissue homogenates from cerebral cortex (M1) and gastric fundus (M2). The ratio of IC50 values of the test compounds in the two different tissues was taken as a measure of M1 receptor selectivity. Several derivatives, especially those with flexible side chains, i.e. high degree of freedom of rotation around single bonds, proved to be nonselective. Among semirigid compounds only those containing 6-membered ring systems (11, 13, 14, and 15) showed significant M1 selectivity. Principles of structure-activity and structure-selectivity are discussed.