This study investigated the immunohistochemical distribution of collagen type I and laminin during remodeling of the uterine extracellular matrix in response to embryo implantation in the rat. Collagen type I was found to be virtually absent around the embryo on the evening of day 5 of pregnancy. On days 6 to 8 of pregnancy the areas of primary and secondary decidualized tissue contained very little collagen in contrast to the outer nondecidualized stroma and myometrial tissues in which the staining patterns did not appear to alter. Day 8 of pregnancy was also notable for the appearance of collagen type I at the site of the developing placenta. Localization of laminin corresponded to areas of basement membrane and was associated with the redistribution of blood vasculature during implantation. By day 7 laminin staining was diminished in the basal areas of the lumenal epithelium around the implanting embryo. Laminin was also located in a punctate fashion at the margins of the primary decidual cells on day 6 of pregnancy, but by days 7 and 8 this staining pattern was no longer evident. This study has provided further evidence for a decline in a major fibrillar collagen during natural decidualization and also revealed a localized and transient expression of laminin in association with the differentiation of cells during primary decidual formation.