To compare the expression of endothelial nitrous oxide synthase (e-NOS), which plays a major role in vasoconstriction, in preeclampsia with that in the normotensive placenta and to identify whether or not e-NOS actually influences the pathogenesis of preeclampsia, we have studied the distribution and intensity of the endothelial isoform of NOS expression in preeclampsia (n = 121) and normal (n = 14) pregnancies. Archival paraffin-embedded tissues were immunostained with a polyclonal e-NOS antibody. E-NOS was stained predominantly in the endothelial cells of the umbilical cord and along the surface of the villi, especially within the cytoplasms of the syncytiotrophoblasts with a granular and punctuated pattern, but not in the trophoblastic island or in the intravillous capillaries. The degree of e-NOS expression in pregnancy-induced hypertension (PIH) was significantly lower than in normotensive placenta, and moreover in the patients lacking abruptio. The expression of e-NOS in preeclampsia has some correlation with patient age. In conclusion, the degree of e-NOS expression in cotyledon from PIH is far less than from control, it means a kind of hemodynamic alteration of the fetoplacental circulation in preeclampsia.