We evaluated the antioxidative effect of melatonin on the oxidized low-density lipoprotein (LDL)-induced impairment of nitric oxide (NO) production in human umbilical artery, which may be the prime cause of endothelial dysfunction in pre-eclampsia. Umbilical artery sections with intact endothelium were obtained from healthy pregnant women who were delivered between 37 and 40 wk of gestation. The production of NO in the umbilical arteries was stimulated by adding l-arginine followed by incubation for 60 min. NO concentrations were estimated by measuring nitrite ions (NO2−) using high-performance liquid chromatography. LDL was oxidized by incubation with 5 μM CuSO4 at 37°C for 4 hr, followed by dialysis at 4°C for 24 hr. Prior to the addition of l-arginine, the segments were treated with native or oxidized LDL (0, 50, 100, 200, 400 μg/mL), or were pre-treated with either mannitol (50 mM) or melatonin (20, 100, 500 μM) before adding oxidized LDL. Changes in l-arginine-induced NO2− production were expressed as a percentage of NO2− production at the end of pre-incubation. Treatment with oxidized LDL significantly reduced l-arginine-induced NO2− production (P<0.05), while NO2− production did not change by incubation with native LDL. Pre-treatment with melatonin significantly increased NO2− production that had been decreased by oxidized LDL (P<0.05). Similarly, pre-treatment with mannitol reversed the oxidized LDL-induced reduction in NO2− production (P<0.05). These results indicate that melatonin protects against oxidized LDL-induced inhibition of NO production in the endothelium of human umbilical arteries, most likely through its ability to scavenge hydroxyl radicals.