Atrial natriuretic peptide (ANP) is a potent natriuretic and hypotensive substance whose importance in pregnancy has not been fully elucidated. Because abnormalities of sodium and water balance are known to be involved in the pathogenesis of toxemic pregnancy, the authors attempted to evaluate the role of ANP by measuring its plasma concentration in 16 normal and 22 toxemic pregnant women (gestational age 26-40 wks, mean 34.5 +/- 1.2 and 28-40 wks, mean 36.0 +/- 1.0, respectively). The weight gain during pregnancy was significantly higher in the toxemic group (13.9 +/- 1.0 vs 9.1 +/- 1.0 kg, p less than 0.01). Plasma ANP level in the toxemic group (40.65 +/- 6.17 pg/ml) was significantly (p less than 0.05) increased compared to that measured in the normal pregnancy group (22.46 +/- 4.22 pg/ml). There was no statistical correlation between ANP levels and gestational age or weight gain during pregnancy. The results suggest that ANP may play a role in the regulation of impaired water and sodium homeostasis in toxemic pregnancy.