Novel analogues of cis-N,N,N-trimethyl-(6-methyl-1,4-dioxan-2-yl)methanaminium iodide (2a) were synthesized by inserting methyl groups alternatively or simultaneously in positions 5 and 6 of the 1,4-dioxane nucleus in all combinations. Their biological profile was assessed by receptor binding assays at human muscarinicM1–M5 receptors stably expressed in CHO cells and by functional studies performed on classical isolated organ preparations, namely, rabbit electrically stimulated vas deferens, and guinea pig electrically stimulated left atrium, ileum, and lung strips. The results showed that the simultaneous presence of one methyl group in both positions 5 and 6 with a trans stereochemical relationship with each other (diastereomers 4 and 5) or the geminal dimethylation in position 6 (compound 8) favour the selective activation of M3 receptors. Compounds 4, 5, and 8 might be valuable tools in the characterization of the M3 receptor, as well as provide useful information for the design and development of novel selective M3 antagonists.