OBJECTIVE: Nitric oxide is a potent vasorelaxant produced by endothelial cells. We tested the hypothesis that urinary and perhaps plasma nitric oxide metabolites would be reduced in women with preeclampsia. STUDY DESIGN: Plasma and urine from 14 women meeting strict clinical criteria for the diagnosis of preeclampsia and 20 normal nulliparous women were assayed for the stable metabolites of nitric oxide, nitrate and nitrite. RESULTS: There was no significant difference of plasma concentrations of nitrate and nitrite between women with preeclampsia and women with normal pregnancies (32.7 ± 3.1 vs 25.8 ± 2.4 μmol/L). Plasma creatinine levels were elevated in women with preeclampsia (0.85 ± 0.09 vs 0.66 ± 0.02 mg/dl, p < 0.01), indicating a reduced glomerular filtration rate. Urine concentrations of nitrate and nitrite normalized by creatinine excretion were significantly lower in women with preeclampsia compared with normal pregnant women (0.37 ± 0.06 vs 0.69 ± 0.11 μmol of nitrite per milligram creatinine, p < 0.05). CONCLUSIONS: Our study using concomitant measurement of plasma and urine nitrate and nitrite suggests a reduced production of nitric oxide in women with preeclampsia compared with normal pregnant women. (AM J OBSTET GYNECOL 1996;174:1008-13.)