Peripheral venous plasma concentrations of immunoreactive atrial natriuretic peptide (irANP) were studied longitudinally in 12 women at the 12th, 24th, and 36th week of pregnancy as well as 3-5 days and 3 months post partum. Serial measurements of maternal hemodynamics were performed simultaneously with blood sampling for irANP determination. With advancing pregnancy there were significant increases (p less than 0.001) in cardiac output, stroke volume and heart rate, while total peripheral vascular resistance decreased (p less than 0.001). All these changes were normalized 3 months post partum. Plasma irANP increased (p less than 0.05) from 23.2 +/- 1.3 pM/l at week 12 to 25.9 +/- 1.5 pM/l at week 36 of pregnancy, and fell significantly (p less than 0.01) to 20.5 +/- 1.1 pM/l 3 months post partum. Changes in plasma irANP appear to be related to changes in maternal central hemodynamics. The changes in ANP release probably represent one of several mechanisms that maintain circulatory and volume homeostasis during normal pregnancy.