Marinobufagenin (MBG) is an endogenous mammalian cardiotonic steroid that is involved in the inhibition of the sodium pump Na(+)/K(+)-ATPase. Increased plasma levels of MBG have been reported in patients with volume expansion-mediated hypertension and preeclampsia. We have recently demonstrated that MBG impairs both the proliferation and growth factor-induced migration of human first trimester cytotrophoblast (CTB) cells, crucial for proper placental development. However, the intracellular signaling mechanisms regulating the MBG-induced impairment of CTB differentiation, migration and invasion are unknown. The human extravillous CTB cell line SGHPL-4 was utilized for this study. The phosphorylation of MAP kinase protein ERK1/2 was evaluated by Cellular Activation of Signaling ELISA (CASE) in control CTB cells and those treated with MBG. MBG at concentrations of 10 and 100nM inhibited CTB cell proliferation, migration and invasion (60%, 50% and 50%, respectively). MBG also caused a significant decrease in the phosphorylation of ERK1/2. In addition, MBG decreased proliferation, migration, and ERK1/2 activity in another motile cell line, CHO cells. Another sodium pump inhibitor, ouabain, similarly decreased proliferation and ERK1/2 activity in CTB and CHO cells. These data suggest that the changes observed in cell function may be mediated by inhibition of Na(+)/K(+)-ATPase. We demonstrate that the MBG-induced impairment of CTB cell proliferation, migration and invasion is associated with decreased ERK1/2 activity which may be mediated by inhibition of Na(+)/K(+)-ATPase.