In early pregnancy, human extravillous trophoblasts (EVTs) invade and remodel maternal arteries. We have previously demonstrated that CCR1 is expressed on perivascular/endovascular trophoblasts and that CCR1 ligands promote EVT migration. In this study, we examined the physiologic roles of platelet-derived chemoattractants on EVT invasion. By immunohistochemistry, maternal platelets were localized among endovascular trophoblasts within the lumen of spiral arteries. Extracellular matrices (ECMs) were also detected among endovascular trophoblasts and platelets, suggesting that the platelets in these arteries were activated by ECMs. In vitro, platelets attached to EVTs isolated from human villous explant cultures and expressed P-selectin on the cell surface. Platelets significantly enhanced migration of EVTs without affecting proliferation of EVTs or secretion of MMP-2 or MMP-9. The invasion-enhancing effect of platelet-derived culture medium on EVTs was neutralized by anti-CCR1 antibody. Heat treatment completely abrogated the invasion-promoting effects of platelet-derived culture medium, but charcoal stripping did not. Platelets also induced endovascular trophoblast-like morphologic changes and integrin alpha1 expression in EVTs during 48-hour culture. These findings suggest that maternal platelets activated in the spiral arteries can regulate trophoblastic vascular infiltration and differentiation by releasing various soluble factors.