This study defines the pharmacologic characteristics of LGR7 and LGR8, the receptors for H2 relaxin and INSL3 respectively, and determines the relative activity of relaxin-related peptides. We show, for the first time, the availability of two binding sites at LGR8 and confirm the presence of two sites at LGR7. Relaxin-related peptides had differing rank orders of affinity and potency at LGR7 and LGR8, but chimeric receptors were highly similar to their ectodomain-origin native receptors. The high-affinity site on the ectodomain coupled efficiently to cAMP production, whereas the low-affinity site in the transmembrane region coupled with decreased efficiency.