Relaxin-3, the most recently identified member of relaxin/insulin family, is an agonist for leucine-rich repeat-containing G protein-coupled receptor (LGR)7, GPCR135, and GPCR142. LGR7 can be pharmacologically differentiated from GPCR135 and GPCR142 by its high affinity for relaxin. Selective ligands that specifically activate GPCR135 or GPCR142 are highly desirable for studying their functional roles. We have created chimeric peptides that consist of the B-chain of human relaxin-3 in combination with various A-chains from other members of the relaxin/insulin family. Pharmacological characterization of these chimeric peptides indicates the A-chain from relaxin-1, relaxin-2, insulin-like peptide (INSL)3, and INSL6 does not change the pharmacological properties of relaxin-3 significantly. In contrast, substitution of the relaxin-3 A-chain with the A-chain from INSL5 results in a chimeric peptide that selectively activates GPCR135 and GPCR142 over LGR7. This study demonstrates that the A-chains among some of the insulin/relaxin family members are pharmacologically exchangeable. The relaxin-3/INSL5 chimeric peptide is a potential tool to study in vivo function of GPCR135. In addition, because of the substitution of a very hydrophobic peptide (the A-chain of relaxin-3) with a very hydrophilic peptide (the A-chain from INSL5), the radiolabeled (125)I-relaxin-3/INSL5 chimera is a suitable ligand (high-affinity, low-nonspecific binding) for receptor autoradiographic studies on tissue sections.