A trial natriuretic peptide (ANP) is a potent natriuretic, diuretic and vasoactive compound secreted by the atria of the heart. A large body of evidence indicates that the degree of atrial stretch or wall tension is a predominant factor in the regulation of ANP secretion.1 The role of ANP during pregnancy is still controversial. Cusson et al2 described an increase in the circulating levels of immunoreactive α-natriuretic factor during normal pregnancy, whereas others3,4 did not find an increase in ANP concentrations. However, measurements during labor and the early puerperium have not yet been reported. Pregnancy is accompanied by an increase in plasma volume, cardiac output and stasis of blood in the lower extremities. During labor and delivery important hemodynamic changes take place, with a further increase in cardiac output.5 After the delivery there is a mobilization and excretion of the retained water and salt accompanied by a diuretic phase. Our hypothesis was that the hemodynamic changes during labor and delivery may stimulate the release of ANP which, in turn, could play a role in the loss of the extra water and salt. The aim of the present study was to examine the pattern of circulating ANP levels before, during and after labor, and during the early puerperium.