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The conditions for coronary steal were determined using the two vasodilators - dipyridamole and nitroglycerin - in anesthetized dogs after ligation of the left anterior descending coronary artery (LAD). Previous studies have shown that when the nonligated coronary arteries are normal, collateral flow increases after dipyridamole. This study utilized a model in which the distal LAD was ligated and the proximal LAD and left circumflex (LC) arteries were stenosed. Heart rate and blood pressure were kept constant. In 20 dogs, 1-1.5 mg/kg dipyridamole caused a decrease in flow to the ischemic region as measured by radioactive microspheres (0.19 to 0.14 ml/min/g, P = 0.05) while flow increased four-fold to surround- ing nonischemic myocardium. The decrease in collateral flow was confined to the epicardial half of the ischemic region (0.26 to 0.14 ml/min/g, P < 0.001) and was associated with an increase in ZST from 30.9 to 44.7mV (P < 0.01). In five dogs nitroglycerin, 5,Ag/kg/min, produced no significant changes in collateral flow or flow to other parts of the LV, and 2ST was unchanged. Vasodilatorinduced coronary steal therefore appears to require 1) an arteriolartype dilator like dipyridamole and 2) stenoses of the arteries supplying collateral flow to the ischemic region. The steal phenomenon is probably caused by a decrease in pressure distal to the stenoses in these vessels, resulting in reduced driving pressure for collateral flow.