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The effect of dietary sodium manipulation was studied In 27 normal subjects and 19 patients with borderline hypertension. Sodium-loading caused an Increase in blood pressure (BP) in 14 of 19 borderline hypertensive patients but in only 4 of 27 normal subjects (X* = 13.85, p < 0.01). In the normal subjects, a 10 mEq sodium diet resulted in a fall in supine mean blood pressure (MBP) of 43% after 4 days (p < 0.05). Cardiac index (CI) measured by echocardiography fell by 4.0%, while total peripheral resistance (TPR) remained unchanged. After the subjects had followed a 200 mEq sodium diet for 4 days, CI rose 6.9%, TPR fell 13%, and MBP fell 3.9%. Six normal subjects also receired a 400 mEq sodium diet, which resulted in a 16% increase in MAP (p < 0.01). This was accompanied by an 11.1% increase in CI and an 11.5% increase in TPR. When subjected to sodium depletion, the 19 hypertensive subjects displayed a similar 4.7% fall in MBP (p < 0.01), a 14.1% fall in CI (p < 0.05) and a 17.1% increase in TPR. The bypertensire patients railed in their response to sodium repletion, 14 displayed a rise in diastolic blood pressure (DBP) (average 8.4%,/? < 0.01) when receiving a 200 mEq sodium diet, while five displayed a fall (average: 13.0%). Those individuals whose DBP fell with sodium repletion had a fall in TP.R of 20.5% (p < 0.05) and rise in CI of 143% (p < 0.025) similar to the changes seen in normal subjects. Those displaying a rise in DBP usually had a rise in CI (9 patients, average rise 20.9%, p < 0.05) while TPR fell 24.5%. The other five patients whose BP rose with salt repletion (10.3%) displayed a 21.9% rise in TPR and a 14.9% fall in CI. We conclude that many patients with borderline hypertension differ from normal subjects by displaying a BP increase due to a disproportionate rise in CI and an inadequate fall in TPR in response to an acute Increase in dietary sodium. (Hypertension 2: 506-514, 1980)