Recent molecular and pharmacologic studies have identified LGR8, a member of the leucine-rich repeat-containing G-protein-coupled receptor family, as a cognate receptor for insulin-like peptide-3 (INSL3). LGR8 mRNA has been detected in various tissues, but the precise roles of these INSL3-LGR8 systems are unknown. In this study we first investigated the presence and cellular localization of LGR8 mRNA in both adult and developing rat kidney and subsequently examined the possible role of INSL3-LGR8 signaling in cultured mesangial cells. LGR8 mRNA was detected in the kidney by polymerase chain reaction and localized by in situ hybridization in mature glomerular mesangial cells within the renal cortex, with highest levels detected at embryonic day 18 and lowest levels in adult kidney. Synthetic INSL3 inhibited the proliferation of mesangial cells in primary culture, indicating the presence of functional LGR8 on these cells. These findings suggest that INSL3/LGR8 signaling may be involved in the genesis and/or developmental maturation of renal glomeruli and in the regulation of mesangial cell density in the adult kidney.