A method is descr. for measuring changes in peripheral resistance in the rat hind limb following intra-femoral injns. of vasoactive substances. The aorta is cannulated by a polyethylene tube with side arm leading to a constant pressure bottle, the proximal aorta occluded and substances to be tested injected into the side arm, and time of transit of a bubble between 2 points on the side arm at constant applied pressure is recorded. The aorta is then unclamped and the vascular bed again irrigated by the animal''s own blood. The method is less sensitive than isolated artery prepns., but has the advantage of being more physiological and more stable. The threshold response to norepinephrine occurs at .005 mg. total dose, about 1/7 the amt. necessary to raise the blood pressure. Reproducible effects can be secured for hours in good prepns. and there is an approx. linear dose-response relation up to 0.025 or more mg. total dose. The principle may be applied, to larger animals and other vascular beds and can also be used to determine aortic pressures and vascular end pressures.