In nine patients with lower extremity symptoms of arterial insufficiency, potassium chloride K 43 was injected intravenously during rest, reactive hyperemia, or exercise. Decreased radioactivity in muscle tissue was observed to correspond with symptoms, physical findings, Doppler ultrasound pressures, and angiographic findings in all six who had the procedure. Studies following surgical endarterectomies in two, a bypass procedure in one, and exploration without attempted reconstruction in one showed good correlation with postoperative symptoms, physical findings, and pressure measurements using Doppler ultrasound in three of four patients. The fourth patient showed no change on the postoperative study despite clinical improvement. These findings suggest that intravenously administered radioactive potassium provides a noninvasive means for demonstrating the perfused muscle mass of the extremities with delineation of ischemic areas. The risk of arterial puncture is eliminated, and the ability to visualize perfusion patterns during exercise is unique to this method.