Abstract : Preeclampsia, which is potentially more ominous with adverse pregnancy events that can affect virtually every organ system than other hypertensive disorders. Nitric oxide is one of the most important biologicals signaling molecules that modulate myogenic tone and flow-mediated responses in the resistance vasculature of the uterine circulation and placenta. Nitric oxide is generated by endothelial nitric oxide synthase. To measure the levels of nitric oxide production in patients with pre-eclampsia, and compare these with the levels in women with normal healthy pregnancy without any pre-existing medical problems; and also, to compare these levels between the patients with pre-eclampsia according to it is severity. The study design as a comparative case-control study was performed in Al-Zahraa Teaching Hospital/ Department of Obstetrics and Gynecology in Najaf- city- Iraq, during the period from 15th February till 30th December 2022. In this study, after verbal consent has been taken from all participants, 100 pregnant women, which include (50 cases) of pregnant women which subdivided into (30), complicated by mild pre-eclampsia, (20) complicated by severe pre-eclampsia, and (50 controls) healthy pregnant women, without any pre-existing medical problems. All these pregnant women who have attended to the department of obstetrics and gynecology either for follow-up, or specific reason, or for complication has been developed, during 28 to 40 weeks of pregnancy. Nitric oxide (NO) is a highly unstable biological free radical with a small molecule, which can easily be oxidized to form nitrate (NO-3) and nitrite (NO-2) in the human body and an aqueous solution. The mean serum nitric oxide levels were found in cases groups (Mean±SD 0.021±0.008) in severe preeclampsia, (Mean±SD 0.042±0.009) in mild preeclampsia, and (mean ± SD 0.094 ± 0.009) in control groups, which was significantly (P< 0.040) higher among pregnant women with the control group, while mean serum nitric oxide was significantly lower among pregnant women with severe and mild preeclampsia. There is a partial negative correlation between serum nitric oxide concentration (NO) and means arterial pressure (MAP) (R2 Linear =0.508), a highly significant (p < 0.000), meaning there is a decrease in the concentration of nitric oxide in study groups of severe and mild preeclampsia correlated partial negatively with an increase in mean arterial pressure of these study groups. As the pregnancy progressed there was a decrease in nitric oxide levels in preeclampsia related to it is the severity.