On the basis of the long-known prototypic pharmacophore 3-(1H-imidazol-4-yl)propylguanidine (SK&F 91486, 2), monomeric, homodimeric, and heterodimeric bisalkylguanidine-type histamine H2 receptor (H2R) agonists with various alkyl spacers were synthesized. Aiming at increased H2R selectivity of the ligands, the imidazol-4-yl moiety was replaced by imidazol-1-yl, 2-aminothiazol-5-yl or 2-amino-4-methylthiazol-5-yl according to a bioisosteric approach. All compounds turned out to be partial or full agonists at the h/gp/rH2R. The most potent analogue, the thiazole-type heterodimeric ligand 63 (UR-Po461), was a partial agonist (Emax = 88%) and 250 times more potent than histamine (pEC50: 8.56 vs 6.16, gpH2R, atrium). The homodimeric structures 56 (UR-Po395) and 58 (UR-Po448) exhibited the highest hH2R affinities (pKi: 7.47, 7.33) in binding studies. Dimeric amino(methyl)thiazole derivatives, such as 58, generated an increased hH2R selectivity compared to the monomeric analogues, e.g., 139 (UR-Po444). Although monomeric ligands showed up lower affinities and potencies at the H2R, compounds with a short alkylic side chain like 129 (UR-Po194) proved to be highly affine hH4R ligands.