Different methods of selective vasodilation have been described. One method is reactive hyperemia, which occurs when an arterial occlusion is released, and another method involves injection of vasodilator drugs. Since both arteriography and pressure measurements are often performed during hyperemia, it is important to know whether these two methods differ in the degree or duration of hyperemia achieved. We measured reactive hyperemia in the hind limb of five anesthetized mongrel dogs after arterial occlusion times between 20 seconds' and 20 minutes' duration and after selective intra-arterial injection of 5, 10, and 20 cc of Renografin-76. Only 5 cc of Renografin-76 created greater and longer lasting hyperemia than the maximum reactive hyperemia achieved after 10 minutes of arterial occlusion. For this reason, contrast material-induced hyperemia appears to be preferable to reactive hyperemia as an aid to peripheral arteriography and as an aid to the evaluation of aortoiliac stenoses.