1. Serum nitric oxide (NO) levels (determined by its products of oxidation) were assessed in nonpregnant women, normal pregnant women and patients suffering from mild pre-eclampsia (MPE), severe pre-eclampsia (SPE), chronic hypertension (CHT) and CHT with pre-eclampsia (CHT+PE). The levels of NO products were significantly reduced during pregnancy in MPE (P < 0.001), CHT+PE (P < 0.01) and SPE (P < 0.05). Significant reductions of NO products were also observed in puerperium (P < 0.001) in all groups except CHT+PE (P < 0.05). 2. In normal pregnancy, three events were related to NO levels: (1) negative correlations were found between the levels of nitrite (r = −0.73, P = 0.0003), nitrate (r = −0.53, P = 0.017) and the number of weeks of gestation; (2) in the caesarean section group, the levels of NO at puerperium were significantly lower (P < 0.05) than those during pregnancy; and (3) there was a significant reduction in NO levels in the pregnant women carrying male fetuses as compared with female fetuses (P < 0.05). 3. In SPE, the patients with a family history of hypertension had lower levels of NO compared with the patients without such a history (P < 0.05). 4. A negative correlation was observed between systolic blood pressure, diastolic blood pressure and NO levels in MPE (r = −0.62, P = 0.013 and r = −0.68, P = 0.0049 respectively) and SPE (r = −0.72, P = 0.004 and r = −0.53, P = 0.037 respectively). 5. In SPE, positive correlations were observed between platelet count and nitrite (r = 0.67, P = 0.006) and nitrate levels (r = 0.56, P = 0.028). 6. In MPE, patients with anti-hypertensive treatment showed significantly (P < 0.05) higher levels of NO compared with the non-treated patients. 7. NO may be important in the physiopathology of hypertension during pregnancy, although several factors may affect its levels.