OBJECTIVE: Our purpose was to compare the expression of endothelial nitric oxide synthase in the placenta and umbilical cord of preeclamptic placenta with that of the normotensive placenta. STUDY DESIGN: We compared placental endothelial nitric oxide synthase expression in preeclamptic (n = 3) with that in normal (n = 3) pregnancies. Frozen sections of umbilical cords, chorionic plate vessels, and terminal villi were immunostained with a monoclonal endothelial nitric oxide synthase antibody (H32). RESULTS: No difference in endothelial nitric oxide synthase immunostaining in the endothelium of the umbilical cord artery and vein, chorionic plate vessels, and stem villous vessels was found between preeclamptic and normotensive pregnancies. In contrast, in the preeclamptic placentas endothelial nitric oxide synthase immunostaining was seen in the small terminal villous vessels with underlying smooth muscle layer. In the syncytiotrophoblast endothelial nitric oxide synthase immunostaining appeared primarily apical in location and diffuse in distribution in the preeclamptic placentas but primarily basal and punctate in the normotensive placentas. CONCLUSIONS: Differences in endothelial nitric oxide synthase expression in terminal villous vessels and in syncytiotrophoblast may be a result of vascular alterations or damage that take place in the placenta in preeclampsia. These differences may alter the regulation of blood flow in the fetal and maternal placental vasculatures in preeclampsia. © 1995.