Appropriate integrin expression appears to be necessary for successful implantation of human embryos and varies considerably among species. The present study was undertaken to determine the distributions of integrin subunits α₁, α₃, and α₆ as well as the extracellular matrix (ECM) components collagen IV and laminin in implanting bovine trophoblast and endometrium. Immunohistochemical staining of cryostat sections prepared from nonpregnant endometrium, of preattachment through to early villus development pregnant endometrium (Days 18, 21, 24, and 30), and of isolated trophoblast binucleate cells was performed. Trophoblast down-regulated the integrin α₁ subunit as attachment proceeded, whereas reactivity scores for α₆ antibody tended to increase from Day 18 through 24 and remained high. A subpopulation of trophoblast binucleate cells expressed the α₃ integrin subunit. Uterine epithelium constitutively expressed α₃ and α₆ integrin subunits, but the α₁ subunit was down-regulated as the luminal epithelium was modified. Collagen IV and laminin reactivity increased in the basal lamina and underlying subepithelial stroma as pregnancy proceeded. The results suggest that binucleate cell fusion with the maternal epithelium initiates integrin and ECM changes in the subepithelial stroma.