Nitric Oxide (NO) is a potent vascular endothelial cell derived vasorelaxant with important effects on vascular tone. This study was initiated to clarify the role of urinary metabolites of nitric oxide (NOx) during pregnancy. Twenty four normal pregnant women and 12 patients with preeclampsia were studied. Urinary NOx levels were determined with the Greiss reagent and spectrophotometry methods after enzymatic reduction of nitrate to nitrite in the samples. Urinary excretion of nitric oxide (NOx) were elevated in normal pregnant women and decreased in preeclamptic women. A significant correlationship was observed in the urinary excretion of cyclic guanosine monophosphate (cGMP) level in between the normal and preeclamptic puerperial groups (p < 0.05). Positive correlationships were found between urinary excretion of nitrate level and either of fractional excretion of sodium (FENa) (r = 0.702, p < 0.005) or fractional excretion of calcium (FECa) (r = 0.570, p < 0.04) in normal and preeclamptic subjects. Also a positive correlationship was observed between the level of urinary NOx excretion and high density lipoprotein cholesterol (HDL-cholesterol) (r = 0.681, p < 0.005). Our results suggest that nitric oxide may play an important role in the mechanism to modulate circulatory physiology of normal and preeclamptic pregnant women.