There is evidence that oxidative stress accompanies preeclampsia and plasma ascorbate concentrations are reported to be decreased in the disorder. We tested the hypothesis that an ascorbate-oxidizing activity is increased in plasma from women with preeclampsia relative to normal pregnancy. Electron paramagnetic resonance (EPR) spectroscopy was used to determine (1) plasma functional reserves of ascorbate and total thiols, (2) temporal changes in ascorbate and thiol concentrations during incubation of whole blood in vitro, and (3) ascorbate radical signal kinetics in plasma after equalization of ascorbate concentrations. High-pressure liquid chromatography (HPLC) was used to measure plasma alpha-tocopherol. Ascorbate concentrations were 50% lower in preeclampsia relative to normal pregnancy plasma but thiols and alpha-tocopherol did not differ. The elapsed time prior to half-consumption of plasma ascorbate was decreased approximately three-fold during incubation of whole blood from preeclamptics. No concomitant decrease in thiols was evident. The initial ascorbate radical signal amplitude was greater in preeclampsia plasma and then, in contrast to normal pregnancy plasma, decreased progressively. The iron chelator, deferoxamine had no effect on plasma ascorbate radical formation. We conclude that an ascorbate-oxidizing activity is increased in preeclampsia plasma which might contribute to vascular dysfunction in the disorder.