Background Technetium-99m-labeled methoxyisobutyl isonitrile (Technetium-99m-sestamibi) is a myocardial perfusion agent that clears slowly from the myocardium. This study evaluates the early and late myocardial distributions of Technetium-99m-sestamibi and Thallium-201 in the presence of low-flow ischemia to determine whether Technetium-99m-sestamibi demonstrates rest "redistribution."Methods and Results Low-flow ischemia was produced in 18 anesthetized, open-chest dogs by partial occlusion of the left anterior descending coronary artery. Dogs were injected intravenously with Technetium-99m-sestamibi, Thallium-201, and radiolabeled microspheres during sustained low-flow ischemia. The hearts were excised either 20 minutes (group 1, 10 dogs) or 2.5 hours (group 2, 8 dogs) after injection for gamma well counting to evaluate the early and late myocardial distributions of these radiotracers, relative to microsphere flow. The early myocardial distributions of Technetium-99m-sestamibi and Thallium-201 were comparable and correlated with the flow deficit (group 1). We observed a significant difference in myocardial Thallium-201 (P=.005) and Technetium-99m-sestamibi (P<.0001) activities between groups 1 and 2 dogs relative to flow, suggesting some redistribution of both tracers. Myocardial slices were imaged postmortem with a gamma camera, and Technetium-99m-sestamibi defect intensity was quantified. There was excellent correlation (r=.97) between the early relative Technetium-99m-sestamibi defect intensity on postmortem images and the flow deficit (group 1). Among group 2 dogs, the correlation was good (r=.87), but the Technetium-99m-sestamibi defect was less severe than the flow deficit, again suggesting redistribution.Conclusions The myocardial distributions of Technetium-99m-sestamibi and Thallium-201 early after injection are comparable and proportional to flow. Under conditions of sustained low flow, there was detectable rest "redistribution" of Technetium-99m-sestamibi verified by both gamma well counting and high-resolution postmortem imaging of myocardial slices. Whether this degree of Technetium-99m-sestamibi rest redistribution will be detectable by serial clinical imaging remains uncertain. Nevertheless, these data suggest that imaging should be delayed after the resting injection of Technetium-99m-sestamibi when assessing myocardial viability in the presence of a critical stenosis. (Circulation. 1994;89:2332-2341.)