1. Changes in plasma noradrenaline levels and heart rate were used as measures of baroreflex sensitivity in six hypertensive subjects given serial incremental doses of sodium nitroprusside (intravenously) to lower blood pressure.

2. The rises in both heart rate and plasma noradrenaline concentration were linearly related to the decrement in blood pressure and inversely related to the severity of the hypertension.

3. A positive correlation between rise in heart rate and rise in plasma noradrenaline was found for each subject. With increasing severity of hypertension, a greater increase in heart rate occurred for each increment in plasma noradrenaline concentration.

4. Baroreflex sensitivity can be assessed by relating changes in heart rate to change in arterial pressure; however, this method does not distinguish the relative contributions of the vagal and sympathetic components of the autonomic neural response or variations in the chronotropic response to sympathetic stimulation.

5. Changes in plasma noradrenaline levels in response to graded reductions in blood pressure may be a more appropriate measure of baroreflex sensitivity than the methods currently used in clinical investigation.

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