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To clarify the effects on angiotensin II and blood pressure on plasma atrial natriuretic peptide in pregnancy, angiotensin II was infused in 37 healthy, normotensive pregnant women in week 32 of gestation. Levels of atrial natriuretic peptide were determined with the women in a sitting position 3 days before the test and in the left lateral recumbent position immediately before and after the test. Levels of atrial natriuretic peptide were not influenced by parity or salt intake. They were found to be significantly higher in the left lateral recumbent position than in the sitting position. There was also a significant increase in atrial natriuretic peptide levels during angiotensin II infusion. A dose of angiotensin II less than 1.5 ng/kg/min did not affect atrial natriuretic peptide levels. Rise in atrial natriuretic peptide levels was minimally correlated with rise in mean arterial pressure, as caused by acute vasoconstriction from angiotensin II infusion. Therefore it is suggested that, in pregnancy, the response of atrial natriuretic peptide to angiotensin II is mediated not only by changes in blood pressure but also by angiotensinergic receptor activity in the atria.