3-(2-Amino-4-methylthiazol-5-yl)propyl-substituted carbamoylguanidines are potent, subtype-selective histamine H<sub>2</sub> receptor (H<sub>2</sub>R) agonists, but their applicability as pharmacological tools to elucidate the largely unknown H<sub>2</sub>R functions in the central nervous system (CNS) is compromised by their concomitant high affinity toward dopamine D<sub>2</sub>-like receptors (especially to the D<sub>3</sub>R). To improve the selectivity, a series of novel carbamoylguanidine-type ligands containing various heterocycles, spacers, and side residues were rationally designed, synthesized, and tested in binding and/or functional assays at H<sub>1-4</sub> and D<sub>2long/3</sub> receptors. This study revealed a couple of selective candidates (among others <b>31</b> and <b>47</b>), and the most promising ones were screened at several off-target receptors, showing good selectivities. Docking studies suggest that the amino acid residues (3.28, 3.32, E2.49, E2.51, 5.42, and 7.35) are responsible for the different affinities at the H<sub>2</sub>- and D<sub>2long/3</sub>-receptors. These results provide a solid base for the exploration of the H<sub>2</sub>R functions in the brain in further studies.