To define the time course of redistribution of thallium-201 (201Tl), ischemia was induced in seven pigs by temporary occlusion of the circumflex coronary artery. After 1 1/2 min of occlusion 201Tl and labeled microspheres were injected into the left atrium. Flow was re-established 4 min after occlusion. Prior to reflow, the relative activities of 201Tl and microspheres in the ischemic area were similar, but as early as 5 min after reflow the relative 201Tl activity was considerably higher than the relative microsphere activity and from 15 to 105 min after reflow, relative 201Tl activity (averabe 69% of that in normal myocardium) continued to be higher than relative microsphere activity (average 6% of normal). Myocardial arteriovenous differences for 201Tl were followed sequentially after 201Tl injection in normal dogs and in dogs with temporary coronary occlusions. The results suggested both loss of 201Tl from normal myocardium beginning 10 min after 201Tl injection and increased extraction of 201Tl from the blood pool immediately after release of a transient occlusion. Redistribution of 201Tl therefore begins very soon after relief of myocardial ischemia and even a short delay in initiating myocardial imaging may decrease the sensitivity of the technique for detecting transient ischemia.