Plasma concentration of atrial natriuretic peptide (ANP) was determined in pregnant women with preeclampsia, in normal pregnant and in nonpregnant women by a specific radioimmunoassay. Results did not show important differences between nonpregnant controls and normal pregnant women, but a significant rise was seen in women with preeclampsia compared to nonpregnant controls. Marked interindividual variation was found in all three groups. The mechanism of ANP release may differ between those women with normal pregnancy and those with preeclampsia. It is unclear whether the increased level of ANP in preeclampsia is an effect or a cause of the disease.