The extracellular calcium sensing receptor (CaSR) belongs to the type III family of G-protein-coupled receptors, a family that comprises the metabotropic glutamate receptor and the putative vomeronasal organ receptors. The CaSR plays an important role for calcium homeostasis in parathyroid cells, kidney cells and other cells to directly 'sense' changes in the extracellular calcium ion concentration ([Ca2+]o). The mesangial cells are known to be involved in many pathologic sequences through the mediation of altered glomerular hemodynamics, cell proliferation, and matrix production. In this study, we examined the expression of the CaSR in the mouse mesangial cell lines (MMC, ATCC number CRL-1927). Reverse transcription-polymerase chain reaction (RT-PCR) was perform with CaSR-specific primers, and this was followed by nucleotide sequencing of the amplified product; this process identified the CaSR transcript in the MMCs. Moreover, CaSR protein was present in the MMCs as assessed by Western blot and immunocytochemical analysis using a polyclonal antibody specific for the CaSR. Functionally, [Ca2+]o induced the increment of the intracellular calcium concentration ([Ca2+]i) in a dose-dependent manner. This [Ca2+]i increment by [Ca2+]o was attenuated by the pretreatment with a phospholipase C inhibitor (U73122) and also by a pretreatment with a CaSR antagonist (NPS 2390). The similar results were also obtained in IP3 accumulation by [Ca2+]o. To investigate the physiological effect of the CaSR, the effect of the [Ca2+]o on cell proliferation was studied. The increased [Ca2+]o (up to 10 mM) produced a significant increase in the cell numbers. This mitogenic effect of [Ca2+]o was inhibited by the co-treatment with a CaSR antagonist. From these results, the [Ca2+]o-induced [Ca2+]i elevation in the MMC is coupled with the extracellular calcium sensing receptor. Furthermore, [Ca2+]o produces a mitogenic effect in MMCs.