We determined placental tissue levels, production rates, and secretion rates of isoprostanes for placentas obtained from women with normal pregnancies and women with preeclampsia, a hypertensive disorder of pregnancy. Isoprostanes are markers of oxidative stress that exert biological actions such as vasoconstriction. Placental tissue was rinsed and immediately frozen in liquid nitrogen to determine tissue levels of total and free isoprostane. Placental tissue pieces were also incubated in serum-free DMEM for 48 h at 37 degrees C in 95% air/5% CO(2) to determine production rates. Isolated placental cotyledons were perfused for the determination of secretion rates. All samples were analyzed by EIA for isoprostane using an antibody specific for 8-Iso-PGF(2) (15-F(2t)-IsoP). In addition, medium samples were analyzed for malondialdehyde (MDA), a breakdown product of lipid peroxidation. We found that tissue levels of free isoprostane and total isoprostane (free plus esterified forms) were significantly higher for preeclamptic placentas than for normal placentas. Concentrations of isoprostane and MDA in the medium increased progressively during 48 h of incubation of placental explants. At 48 h of incubation, the mean concentrations of both isoprostane and MDA were significantly higher for the placentas from preeclamptic women than for the placentas from normal pregnant women. Concentrations of MDA were highly correlated with those of isoprostane. Induction of oxidative stress with xanthine plus xanthine oxidase increased placental production of isoprostane by normal tissue to a level similar to that of preeclamptic tissue. Placental secretion of isoprostane was eightfold greater toward the maternal side of the placenta than toward the fetal side, and was increased sixfold on the maternal side and twofold on the fetal side by inducing oxidative stress with t-butyl hydroperoxide. This study presents new information that isoprostanes are formed and secreted by the human placenta and provides convincing evidence that oxidative stress and lipid peroxidation are abnormally increased in placentas of preeclamptic women.