Both relaxin-3 and its receptor (RXFP3, also known as GPCR135) are predominantly expressed in brain regions known to play important roles in processing sensory signals. Recent studies have shown that relaxin-3 is involved in the regulation of stress and feeding behaviors. The mechanisms underlying the involvement of relaxin-3/RXFP3 in the regulation of stress, feeding, and other potential functions remain to be studied. Since relaxin-3 also activates the relaxin receptor (RXFP1, also known as LGR7), which is also expressed in the brain, selective RXFP3 agonists and antagonists are crucial for study of the physiological functions of relaxin-3 and RXFP3 in vivo. The finding that the B chain of relaxin-3 is an agonist for RXFP3 (albeit at low potency) but not RXFP1 suggests that the B chain of relaxin-3 plays a dominant role for RXFP3 binding and activation. Chimeric peptide studies using the B chain from relaxin-3 and the A chains from different members of the insulin and relaxin family have confirmed this hypothesis and led to the generation of R3/I5 (a chimeric peptide with relaxin-3 B chain and INSL5 A chain) as a selective agonist for RXFP3 over RXFP1. Truncation of the C-terminus of the B chain of R3/I5 results in a high-affinity antagonist, R3(BΔ23–27)R/I5, for RXFP3 over RXFP1. R3(BΔ23–27)R/I5 has pA2 values of 9.15 and 9.6 for human and rat RXFP3, respectively, but has no affinity or agonistic activity for the human and rat RXFP1. Ongoing and future in vivo studies using the selective agonist and antagonist for RXFP3 will shed light on the physiological role of the relaxin-3 system.