Remodelling of the extracellular matrix (ECM) occurs during decidualization of the endometrium in mice. Previously we have documented the appearance of large-diameter collagen fibrils around mature decidual cells between day 5 and day 7 of pregnancy. Proteoglycans are important in the regulation of collagen fibrillogenesis, and the present study analysed four members (decorin, biglycan, lumican and fibromodulin) of the family of small leucine-rich proteoglycans (SLRPs) in the uterus from day 1 to day 7 of pregnancy. Decorin was present together with lesser amounts of lumican in the stroma before the onset of decidualization, whereas biglycan and fibromodulin were almost absent. Biglycan and, less significantly, lumican were expressed in decidualized regions of the endometrium, but decorin was absent. Fibromodulin was weakly expressed in the non-decidualized stroma, but only after implantation. Decorin and lumican were strongly expressed in the undifferentiated interimplantation site stroma, whereas biglycan and fibromodulin were expressed only weakly. These results indicate that the SLRP profile of the uterine ECM alters with differentiation of endometrial stromal cells. The large decidual collagen fibrils are thought to arise by lateral association of smaller diameter fibrils. As decorin has been shown to inhibit lateral association of collagen fibrils, its disappearance between day 2 and day 5 of pregnancy may be a prerequisite for the formation of large fibrils in decidua in mice.