Technetium-99m-[2-(1-methoxybutyl) isonitrile] (MBI) is a potential new compound for the scintigraphic imaging of coronary flow. Evaluation in the blood-perfused isolated rabbit heart model showed this compound to have a myocardial uptake comparable to 201Tl and higher than sestamibi. Although the mean +/- s.d. maximum extraction (Emax) and capillary permeability-surface area product (PScap) of 99mTc-MBI (Emax = 0.45 +/- 0.10, PScap = 1.07 +/- 0.47 ml/min.g) were much less than 201Tl (Emax = 0.71 +/- 0.07, PScap = 2.21 +/- 0.76 ml/min.g, p < 0.0001), the net extraction of 99mTc-MBI (Enet = 0.52 +/- 0.10) was only slightly less than the value for 201Tl (Enet = 0.56 +/- 0.10, p < 0.05). There was no significant difference in the myocardial uptake versus flow between 99mTc-MBI and 201Tl. These data indicate that assessment of relative coronary flow based on the myocardial uptake of 99mTc-MBI should give results comparable to 201Tl. Therefore, 99Tc-MBI may have clinical potential as a radiolabeled myocardial perfusion agent.