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 non-ligated 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 surrounding 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 less than 0.001) and was associated with an increase in sigmaST from 30.9 to 44.7 mV (P less than 0.01). In five dogs nitroglycerin, 5 microgram/kg/min, produced no significant changes in collateral flow or flow to other parts of the LV, and sigmaST was unchanged. Vasodilator-induced coronary steal therefore appears to require 1) an arteriolar-type 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.