Preeclampsia is characterized by intense and prolonged vasoconstriction. Rho A-mediated calcium sensitization is central to prolonged contractility of vascular smooth muscle. The aims of this study are (1) to investigate mRNA expression levels of Rho A/Rho kinases in placental tissues from normotensive and preeclamptic women, and (2) to investigate the effects of 2 isoprostanes, 8-iso prostaglandin F-2 alpha (8-iso PGF(2 alpha)) and 8-iso prostaglandin E-2 (8-iso PGE(2)), on small placental and myometrial vessel resistance and to determine if their effects were mediated via the Rho kinase pathway. Real-time reverse transcription polymerase chain reaction for Rho A, ROCK I, and ROCK II was perfomed on total RNA from normotensive and preeclamptic placentae. The effects of 8-iso PGF(2 alpha) and 8-iso PGE(2) (alone and with the specific Rho kinase inhibitor Y-27632) on placental and myometrial vessels (<400 mu m) were measured and compared with control recordings. Rho A mRNA expression levels were significantly higher in placentae from preeclamptic women than in placentae from normotensive women (P < .01). There was no signficant difference in expression levels of ROCK I and ROCK II between both tissue types (P > .05). Both isoprostanes exerted a significant concentration-dependent vasocontractile effect on both vessel types (P < .001). This affect was antagonized by Y-27632 in placental arteries but not in myometrial arteries. Increased Rho A mRNA expression in placentae from preeclamptic women is suggestive of a role for the Rho kinase pathway in the modulation of the placental vasculature in this condition. Isoprostanes exert their vasocontractile effect, in placental vasculature, in part via the Rho kinase pathway.