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We observed alpha-smooth muscle actin (αSMA) expression in rat endovascular trophoblasts (EVasT) and investigated the spatial and temporal expression of smooth muscle (SM) proteins and their potential function in remodeled spiral artery. Rat placentas were examined from gestational day 13 to term, and were immunostained for cytokeratin, αSMA, α heavy chain of SM myosin, myocardin (marker of SM differentiation) and endothelin receptors A and B (ETA, ETB: BQ-123 and BQ-788, respectively). Transverse sections of the modified spiral artery were studied ex vivo for endothelin-1-induced contraction. EVasT expressed SM proteins co-localizing with cytokeratin, confirming their trophoblastic origin, Thin fibers, consistent with actin fibers, were observed by transmission electron microscopy, in the cellular localization of αSMA in EVasT.Functional experiments revealed that addition of 10 -7 M endothelin-1 ex vivo reduced vascular lumen cross section area by 10.1+/-0.9% compared with control. This effect was reduced to only 1.1+/-4.9% in the presence of ETA antagonist, and to 5.34.1% by ETB antagonist, p The expression of proteins involved in SM differentiation, regulation of function and contraction, along with the expression of the endothelin system, suggest that some vascular tone is potentially maintained by endothelin-1 in the rat remodeled spiral artery despite replacement of SM cells by trophoblasts. Endothelin-induced contraction of the modified spiral artery ex vivo is mediated via its receptors, suggesting its role in situations of dysregulation of the vasoactive systems.