Even today, the role of the histamine H<sub>2</sub> receptor (H<sub>2</sub>R) in the central nervous system (CNS) is widely unknown. In previous research, many dimeric, high-affinity and subtype-selective carbamoylguanidine-type ligands such as UR-NK22 (5, pK<sub>i</sub> = 8.07) were reported as H<sub>2</sub>R agonists. However, their applicability to the study of the H<sub>2</sub>R in the CNS is compromised by their molecular and pharmacokinetic properties, such as high molecular weight and, consequently, a limited bioavailability. To address the need for more drug-like H<sub>2</sub>R agonists with high affinity, we synthesized a series of monomeric (thio)carbamoylguanidine-type ligands containing various spacers and side-chain moieties. This structural simplification resulted in potent (partial) agonists (guinea pig right atrium, [<sup>35</sup>S]GTPγS and β-arrestin2 recruitment assays) with human (h) H<sub>2</sub>R affinities in the one-digit nanomolar range (pK<sub>i</sub> (139, UR-KAT523): 8.35; pK<sub>i</sub> (157, UR-MB-69): 8.69). Most of the compounds presented here exhibited an excellent selectivity profile towards the hH<sub>2</sub>R, e.g. 157 being at least 3800-fold selective within the histamine receptor family. The structural similarities of our monomeric ligands to pramipexole (6), a dopamine receptor agonist, suggested an investigation of the binding behavior at those receptors. The target compounds were (partial) agonists with moderate affinity at the hD<sub>2long</sub>R and agonists with high affinity at the hD<sub>3</sub>R (e.g. pK<sub>i</sub> (139, UR-KAT523): 7.80; pK<sub>i</sub> (157, UR-MB-69): 8.06). In summary, we developed a series of novel, more drug-like H<sub>2</sub>R and D<sub>3</sub>R agonists for the application in recombinant systems in which either the H<sub>2</sub>R or the D<sub>3</sub>R is solely expressed. Furthermore, our ligands are promising lead compounds in the development of selective H<sub>2</sub>R agonists for future in vivo studies or experiments utilizing primary tissue to unravel the role and function of the H<sub>2</sub>R in the CNS.