In recent years, several technetium 99m-labeled myocardial perfusion agents have been under investigation to determine their utility in assessing regional myocardial blood flow and cellular viability. 99mTc sestamibi (2-methoxyisobutyl isonitrile), one of the most promising of these agents, is a lipophilic cation that is sequestered largely within mitochondria by the large negative transmembrane potential. Experimental studies have shown that this agent is taken up in the myocardium in proportion to blood flow, but like other diffusible radionuclides underestimates flow at high flow rates. Fractional extraction (Emax) for 99mTc sestamibi is lower than that for thallium 201, but net myocardial uptake is comparable between the two radionuclides. Compared with 201Tl, 99mTc sestamibi does not significantly redistribute. Studies have shown that 99mTc sestamibi uptake after reperfusion, preceded by varying periods of coronary occlusion, reflects the degree of myocardial salvage and viability. 99mTc teboroxime, another promising new perfusion agent, is a boronic acid adduct of technetium dioxime complexes. Emax of 99mTc teboroxime is higher than Emax for 201Tl, but myocardial washout is very rapid (half-life = 21 minutes). Myocardial uptake is proportional to regional flow as found with 99mTc sestamibi and 201Tl.