1. The compliance of the resistance vessels in the anterior tibial muscle was compared in normotensive and hypertensive subjects by measuring the increase in muscle blood flow produced by an increase in vascular transmural pressure.

2. The transmural pressure was increased by exposing the leg to a pressure of 50 mmHg below atmospheric pressure. Muscle blood flow was measured by means of the local 133xenon wash-out technique, and the measurement was performed during reactive hyperaemia to avoid reflex vasoconstriction when the transmural pressure was augmented.

3. At ambient pressure, muscle blood flows were equal in the two groups. However, when the subatmospheric pressure was applied the blood flow increased twice as much in the normotensive as in the hypertensive subjects. This suggested a smaller compliance of the resistance vessels in hypertensive subjects and supported the concept that there are structural changes of the resistance vessels in arterial hypertension.

4. In a small group of hypertensive patients who were well treated for two years, and had a normal blood pressure, the compliance was reduced to the same extent as in the untreated hypertensive patients, indicating that structural vascular changes are not readily reversible despite appropriate antihypertensive treatment.

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