Similarities of the rat to the human placenta make rat pregnancy models relevant to the study of human gestational diseases. Understanding of species differences is necessary to extrapolate from animal models to humans. 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, alpha heavy chain of SM myosin, non-muscle myosin, Rho proteins, regulators of SM gene expression, myocardin, an early marker of SM differentiation and endothelin receptors A and B (ETA, ETB). Transmission electron microscopy (TEM) was performed. Modified spiral artery rings were studied ex vivo for endothelin-1- induced contraction.EVasT expressed SM proteins co-localizing with cytokeratin confirming their trophoblastic origin from gestational day 13 to term. Thin fibers, consistent with actin fibers, were observed by TEM, in the cellular localization of αSMA in EVasT. Functional experiments revealed that addition of 10(-7) M endothelin-1 ex vivo reduced vascular lumen area by 11.1% ± 1.8% compared with control. This effect was reduced to only 1.0 ± 1.7% with ETA antagonist, and to 5.4 ± 1.7% contraction by ETB antagonist, p < 0.002, for all.The expression of SM proteins in EVasT along with the contractibility of the rat remodeled spiral artery ex vivo, suggest that some vascular tone is potentially maintained by endothelin-1, and may play a role in situations of dysregulation of the vasoactive systems.