Objective: There is a reduction in intravascular volume in patients with preeclampsia and eclampsia. Since the secretion of atrial natriuretic peptide (ANP) by human atrial myocytes is stimulated by increased intraatrial pressure or atrial distention, we sought to determine whether circulating maternal plasma ANP concentrations were lower in patients with severe preeclampsia and eclampsia compared with normal pregnant women. Methods: Fifteen patients with mild preeclampsia, 15 with severe preeclampsia, 15 with eclampsia, and 38 normotensive pregnant women were taken as the control group. All the women were age-matched and in their third trimester. The level of ANP in these patients was measured during the third trimester and in the postpartum period by a specific radioimmunoassay. At the same time, routine laboratory tests for eclampsia and preeclampsia such as uric acid, creatinine, and urine protein levels were obtained. Results: ANP levels were significantly higher in the antepartum periods in women with mild preeclampsia (20.7 ± 6), severe preeclampsia (28.9 ± 9), and eclampsia (26.6 ± 6.5) than those in normotensive pregnant women (15.7 ± 3.5; P < 0.05 and P < 0.001, respectively). As the severity of preeclampsia increased, the mean values of ANP became higher. Conclusions: ANP levels in preeclamptic and eclamptic women were found to be higher than those in normotensive pregnant women. There is a correlation between the severity of toxemia and plasma ANP levels. The mechanism(s) responsible for the elevation requires further investigation.