Background. The reports on plasma concentrations and physiological function of atrial natriuretic peptide (ANP) during pregnancy are conflicting. In a recent prospective study, including 40 healthy primigravidae, we found a highly significant decrease in the plasma concentration of ANP (p‐ANP) during the third trimester and the results indicated that ANP takes part in regulation of blood volume and renal function during pregnancy as in the nonpregnant state. In order to test these results, a study was performed in primigravidae with twin pregnancy to test if the accentuated physiological changes here were followed by a corresponding greater decrease in p‐ANP.Methods. Ten healthy primigravidae with twin pregnancy were examined four times during pregnancy plus 12 weeks after delivery. Each time the following were measured: p‐ANP, aldosterone, renin, blood volume (carbon monoxide), cardiac output (Doppler), blood pressure and sodium excretion. Interdependence of the changes in ANP and in the other parameters was tested using Spearman's rank correlation test on the delta (Δ)‐values (the differences in measurements between investigations). The results were compared to the results obtained during singleton pregnancy using the Mann‐Whitney rank sum test.Results. All pregnant values of p‐ANP during twin pregnancy were lower than 12 weeks after delivery,p<0.01. In the 20th, 28th, and 32nd week p‐ANP was lower in twin pregnancy than in singleton pregnancy,p<0.05. There was a negative correlation between changes in p‐ANP and changes in: a) blood volume, R=‐0.8,p<0.0001, b) aldosterone, R=‐0.66,p<0.0001, c) renin, R=‐0.52,p<0.01, d) cardiac output, R=‐0.68,p<0.0001. There was a positive correlation between changes in p‐ANP and changes in: a) fractional excretion of sodium, R = 0.73,p<0.0001, and b) total peripheral resistance, R = 0.61,p<0.0001.Conclusion. The results suggest that the competitive relationship between ANP and the renin‐aldosterone system in regulating sodium balance and fluid volume is preserved during pregnancy. The vasodilatation during preg