Tooth resorption occurs in 20-75% of cats (Felis catus). The aetiology is not known, but vitamin D is suggested to be involved. Vitamin D acts through a nuclear receptor (VDR) and increases the expression of receptor activator of nuclear factor-kappaB ligand (rankl) and muscle segment homeobox 2 (msx2) genes. Mice lacking the muscle segment homeobox 2 (msx2) gene show decreased levels of rankl, suggesting an interaction among VDR, MSX2, and RANKL. Here, we investigated the expression of VDR, MSX2, and RANKL proteins, and the activity of the VDR-mediated signalling pathway (using the quantitative polymerase chain reaction on VDR target genes), in tooth resorption, and measured the serum levels of vitamin D metabolites in cats. Tooth resorption was categorized into either resorptive or reparative stages. In the resorptive stage, odontoclasts expressed MSX2 and RANKL (100% and 88%, respectively) and fibroblasts expressed VDR and MSX2 (both at 100%), whereas fibroblasts expressed RANKL in only 29% of the sites analysed. In the reparative stage, cementoblasts expressed VDR, MSX2, and RANKL, whereas fibroblasts expressed VDR and MSX2, but not RANKL. The vitamin D status did not differ between the groups, based on the serum levels of 25-hydroxycholecalciferol. However, increased expression of VDR protein, and the relative gene expression levels of 1alpha-hydroxylase and the VDR-target gene, 24-hydroxylase, indicated the involvement of an active vitamin D signalling in the pathophysiology of tooth resorption in cats.