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The possibility that sympathetic pressor responses are modulated by adrenalinemediated facilitation of neuronal noradrenaline release was explored in 17 subjects with borderline hypertension. Infusion of adrenaline, which raised plasma adrenaline by a factor of 8 to 9, augmented the rise in systolic and diastolic arterial pressure induced by standardized cold pressor and isometric exercise tests. The heart rate response to these tests was not affected. When a low dose of propranolol was given on top of the adrenaline infusion before the cold pressor test, the blood pressure response to cold exposure was not different from the response observed when the test was performed during saline infusion. Plasma noradrenaline was higher during adrenaline infusion then during saline infusion, both before and after the cold pressor and isometric exercise tests, and the effect of adrenaline on plasma noradrenaline was antagonized by propranolol. These observations are consistent with the hypothesis that stress levels of circulating adrenaline may amplify sympathetic pressor responses by facilitation of the release of transmitter noradrenaline.