Summary Distensibility of an isolated segment of brachial artery was measured directly in 11 normotensive and 12 hypertensive humans in vivo. The brachial artery segment between the profunda brachial and ulnar collateral branches was exposed by sterile surgical techniques. An indwelling cannula was inserted into the segment from below. The segment was then isolated and gently emptied. A distensibility curve was constructed by adding fluid (T-1824-tinged normal saline at 37°C) to the segment in stepwise increments of 0.05 ml and recording segment pressure at each fluid increment by the use of a 3-way stopcock arrangement connected to the cannula. There was a distinct separation of the pressure-volume relationships of the brachial artery between the two groups of subjects. Curves from the hypertensive subjects were shifted toward the pressure axis, indicating that this vessel is less distensible in hypertensive humans than in humans with normal blood pressure. The mechanisms responsible for this phenomenon were not elucidated. The pressure-volume relationships of the brachial artery segment formed a curvilinear slope with the upward convexity toward the volume axis. Artery segments do not obey Hooke' Law, i.e., they resist further stretch more strongly the more they are stretched.