Atrial natriuretic peptide (ANP) concentrations are typically elevated in hypervolemic states. However, ANP levels have been reported to be increased in the peripartum period in women with preeclampsia, a disorder characterized by central hypovolemia. We postulated that ANP levels are elevated in preeclamptic patients before clinically evident disease. ANP concentrations were determined in three groups: uncomplicated pregnancies, pregnancies complicated by preeclampsia, and non-pregnant reproductive-aged women. The former groups were matched for gestational age at plasma sampling and delivery. The plasma samples, obtained prospectively from each patient during the first, second, and third trimesters and within 72 h postpartum, were frozen before RIA. A significant gestational increase in ANP was noted in both groups of pregnant women, with third trimester levels exceeding first trimester levels (P less than 0.05). Consistent with previous reports, ANP levels were elevated in overtly preeclamptic patients vs. matched controls in the third trimester. The ANP concentration was also significantly increased during the second trimester in women destined to develop preeclampsia. Postpartum ANP values decreased in the preeclamptic group to approach the level in normal patients postpartum. Thus, it appears that the stimuli of ANP secretion differ in uncomplicated and preeclamptic patients. Moreover, an elevation of plasma ANP is detectable before the onset of clinical evidence of preeclampsia.