This study was designed to assess the sensitivity of the monocationic <sup>99m</sup>Tc-hexakis-2-methoxy, 2-methylpropylisonitrile (<sup>99m</sup>Tc-sestamibi) as compared to <sup>201</sup>T1 in the detection of regional myocardial blood flow deficits in dogs with different degrees of stenosis with or without dipyridamole. Regional myocardial distribution of <sup>99m</sup>Tc-sestamibi and <sup>201</sup>T1 was determined in mongrel dogs under different levels of left arterial descending coronary artery (LAD) stenosis (30–45, 60–70 and 100% flow reduction) as monitored with electromagnetic flow probes and radiolabeled microspheres. Both <sup>99m</sup>Tc-sestamibi and <sup>201</sup>T1 distribute in direct proportion to blood flow at all levels of stenosis (r<sup>2</sup> = 0.90–0.99). Quantitatively, both tracers underestimate the resting flow stenosis. The regional myocardial distribution for both <sup>99m</sup>Tc and <sup>20l</sup>Tl was also determined in subcritically stenosed mongrel dogs. This was produced by placing a balloon cuff around the LAD, infusing dipyridamole to peak hyperemic flow and immediately stenosing the LAD to basal flow. Stenosed to normal zone flow ratios were 0.68 ± 0.01 compared to <sup>99m</sup>Tc and <sup>201</sup>T1 ratios of 0.73 ± 0.04 and 0.80 ± 0.05, respectively. These data suggest that <sup>99m</sup>Tc is at least as sensitive as <sup>201</sup>T1 in the detection of different degrees of critical and subcritical coronary artery stenosis.