Immediate exercise hyperemia in humans is contraction intensity dependent: evidence for rapid vasodilation. J Appl Physiol 96: 639-644, 2004. First published October 24, 2003 10.1152/japplphysiol.00769. 2003.-We tested the hypothesis that rapid vasodilation proportional to contraction intensity contributes to the immediate (first cardiac cycle after initial contraction) exercise hyperemia. Ten healthy subjects performed single 1-s isometric forearm contractions at 5,10,15,20, 30, 50, and 70% maximal voluntary contraction intensity (MVC) in arm above heart (AH) and below heart (BH) positions. Forearm blood flow (FBF; brachial artery mean blood velocity, Doppler ultrasound), mean arterial pressure (arterial tonometry), and heart rate (electrocardiogram) were measured beat by beat. Venous emptying (measured with a forearm strain gauge) was already maximized at 5% MVC, indicating that increases in contraction intensity did not further empty the forearm veins. Immediate increases in FBF were linearly proportional to contraction intensity from 5 to 70% MVC in AH (slope ϭ 4.4 Ϯ 0.5%⌬FBF/%MVC). In BH, the immediate increase in FBF demonstrated a curvilinear relationship with increasing contraction intensity and was greater than AH at 15, 20, 30, and 50% MVC (P Ͻ 0.05). Peak changes in FBF were greater in BH vs. AH from 10 to 50% MVC, even when venous refilling was complete (P Ͻ 0.05). These data support the existence of a rapid-acting vasodilatory mechanism(s) at the onset of human forearm exercise. muscle blood flow; muscle pump IN A REST-TO-EXERCISE TRANSITION, muscle blood flow typically increases in a biphasic manner to a steady-state level proportional to muscle metabolic demand (7,12,17). A striking characteristic of this adaptation is the immediate and substantial increase in muscle blood flow after release of the first contraction of exercise (11-13, 18, 21). Because blood flow through a muscle vascular bed is thought to be proportional to 1) the arteriovenous pressure difference across that bed and 2) the vascular conductance of that bed, a number of studies have been conducted to determine whether the muscle pump, vasodilation, or both play an immediate role in the hyperemia at the onset of exercise (9,11,12,21,23). Considerable evidence for a muscle pump contribution exists, but whether the muscle pump is the exclusive contributor or whether a rapid vasodilatory mechanism(s) exists remains controversial.Studies that support an exclusive muscle pump effect have examined the onset of mild or moderate locomotion in dogs or rats walking on a treadmill. These studies demonstrate that the magnitude of the initial rapid adjustment in blood flow is related to speed (contraction frequency) not grade (contraction intensity) (11, 12). More invasive studies have used video microscopy in situ to directly measure the time course of vessel diameter responses to muscle stimulation or direct application of known vasodilators to microvessels (3,8,23). These studies suggest that the onset of vasodilation exhibits a 4-to 6-s d...