1. Twenty-eight healthy non-pregnant women and 28 women in the first or second trimester of pregnancy were studied. They were given an incremental intravenous infusion of either noradrenaline or angiotensin II. Pressor and heart rate responses were documented.

2. Dose-pressor response curves were constructed for the two agents in pregnant and non-pregnant women (n=14 in each group). The regression parameters of slope and intercept were calculated, and were used to derive the variables of dose required to elicit a 10 mmHg rise in systolic or diastolic blood pressure.

3. The pressor response to angiotensin II was diminished in pregnancy, with approximately twice the dose being required to raise the systolic or diastolic arterial blood pressure as in non-pregnant subjects.

4. The systolic pressor response to noradrenaline was slightly diminished in pregnancy, but the diastolic pressor response was unchanged. There were no significant differences between the doses of noradrenaline required to elicit a 10 mmHg rise in systolic or diastolic arterial blood pressure in pregnant or non-pregnant subjects.

5. There was a diminution in the bradycardia evoked in response to both hormones in pregnancy.

6. We conclude that the well-documented pressor insensitivity to angiotensin II during pregnancy is a specific phenomenon, not a manifestation of a generalized reduction in vascular reactivity.

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