The effects of a flow-limiting stenosis on external circumflex coronary arterial diameter during treadmill exercise were studied in 10 instrumented dogs. Coronary arterial diameter was measured by sonomicrometry proximal to the stenosis-producing hydraulic occluder so that the effects of a post-stenotic pressure drop were excluded. With no stenosis, heart rate increased (116±7 to 183±10 beats/min, p<0.001), aortic pressure increased (97±3 to 105±5 mm Hg. p<0.005), circumflex coronary blood flow increased (48±8 to 72±8 ml/min, p<0.001), and circumflex coronary diameter increased (3.82±0.29 to 3.93±0.27 mm, p<0.01). In the presence of a flow-limiting stenosis, heart rate increased (120±6 to 176±9 beats/min, p<0.001), aortic pressure did not change significantly (95±4 to 92±4 mm Hg), circumflex coronary blood flow increased slightly (39±8 to 46±9 ml/min, p<0.005), and circumflex coronary arterial diameter did not change significantly (3.78±0.29 to 3.80±0.28 mm). The stenosis prevented the increase in aortic pressure, blunted the increase in circumflex coronary blood flow (24±4 versus 7±2 ml/min, p<0.005), and prevented the increase in circumflex coronary arterial diameter. Therefore normal coronary arteries dilated during exercise and a flow-limiting stenosis prevented exercise-induced coronary dilation proximal to the stenosis, possibly due to both the failure of aortic pressure to increase and less flow-induced endothelium-dependent dilation.