Thallium-201 was found to be a reliable agent for detecting decreased myocardial perfusion in domestic pigs 1-4 hr after acute coronary occlusion. Substantial variation in myocardial-to-liver count ratios and diagnostic quality was observed in serial images performed in 3 normal pigs, although areas of 1-4 hr-old myocardial ischemia produced by acute circumflex coronary artery ligation in 6 pigs could be reliably detected by in vivo 201Tl imaging. After intravenous 201Tl administration, the animals were sacrified and sections of normal and ischemic myocardium were counted in a scintillation well counter. The activity in the ischemic area in pigs averaged 12% of the activity in the normal area, and varied over a narrow range; in dogs the activity averaged 62% of normal, and varied over a wide range. The pig was a more consistent model than the dog.