Myocardial perfusion imaging after administration of the potent coronary vasodilator ethyl adenosine-5'-carboxylate, which increases flow to normal areas in excess of that to areas supplied by subcritically stenosed vessels, was investigated as a nonischemia-producing stimulus for detecting subcritical coronary stenosis. Preliminary studies in 10 dogs with reactive hyperemia were performed with thallium-201 and potassium-43 to determine which tracer was a better indicator of increased flow. Neither agent was a linear indicator of increased flow caused by reactive hyperemia but thallium-201, because of its imaging characteristics, was selected as a flow indicator after administration of ethyl adenosine. Five dogs were studied after placement of a subcritical stenosis on the left circumflex coronary artery. Strontium-85 microspheres were injected into the left atrium after placement of the stenosis to verify that changes in resting blood flow were only minimal. Thereafter, intravenous administration of ethyl adenosine was followed by injection of chromium-51-labeled microspheres into the left atrium and intravenous administration of thallium-201. The mean ratio of left circumflex to left anterior descending coronary arterial flow was 0.96 +/- 0.16 for the control experiment after subcritical stenosis; after administration of the vasodilator the ratio of activity levels in the two arteries was 0.43 +/- 0.09 with the chromium-51 microspheres and 0.56 +/- 0.07 with thallium-201. Imaging performed in three additional dogs after injection of microspheres in the presence of subcritical stenosis revealed a normal pattern, whereas imaging after administration of the vasodilator and thallium-201 revealed a perfusion deficit. In two additional dogs without subcritical stenosis, thallium was administered after injection of ethyl adenosine to determine that the drug alone did not cause perfusion deficits. The perfusion scans in these two dogs were normal. These studies suggest that a coronary vasodilator and thallium-201 myocardial imaging can be used to detect subcritical coronary stenosis.