GPC-1 (glypican-1) is a cell surface heparan sulfate proteoglycan that acts as a co-receptor for heparin-binding growth factors and members of the TGF-β (transforming growth factor beta-1) family. The function of cell-surface proteoglycans in the reparative dentine process has been under investigation. Gpc-1 was detected with similar frequency as tgf-β1 in the cDNA library using mRNA from the odontoblast-like cell-enriched pulp of rat incisors. The aim of this study was to test our hypothesis that gpc-1 may be related to reparative dentine formation. We examined the expression of this gene during the reparative dentine process, as well as the effect of gpc-1 on odontoblast-like cell differentiation using siRNA (small interfering RNA) to down-regulate gpc-1 expression. Immunohistological examination showed that GPC-1 was expressed in pulp cells entrapped by fibrodentine and odontoblast-like cells as well as TGF-β1. The mRNAs for gpc-1, -3 and -4, except for gpc-2, were expressed during odontoblast-like cell differentiation in pulp cells. The relative levels of gpc-1 mRNA were increased prior to the differentiation stages and were decreased during the secretory and maturation stages of pulp cells. Down-regulation of gpc-1 expression resulted in a 3.9-fold increase in tgf-β1 expression in pulp cells and a 0.3-fold decrease in dspp (dentine sialophosphoprotein) expression compared with control. These results suggested that gpc-1 and tgfβ-1 expression are necessary for the onset of differentiation, but should be down-regulated before other molecules are implicated in the formation of reparative dentine. In conclusion, gpc-1 expression in odontoblast-like cells is associated with the early differentiation but not with the formation of reparative dentine.