The effects of endurance training on coronary vascular resistance, myocardial blood flow, and oxygen consumption during tachycardia, and with adenosine-induced coronary dilation, were studied in a group of exercise-trained dogs. Seven mongrel dogs were conditioned by 8 wk of running on a motor-driven treadmill. Following conditioning, aortic pressure was unchanged in trained compared to a nontrained group, while cardiac output was somewhat lower in the trained group. Left ventricular myocardial blood flow determined by the microsphere technique was not different at rest or with tachycardia in trained vs control animals, and no significant differences in oxygen consumption were observed between trained and nontrained animals under any experimental conditions. Coronary resistance during pacing (NT: 1.00 +/- 0.09, T: 0.79 +/- 0.06 mmHg/ml X min-1 X 100 g-1) in the trained group was similar to the nontrained group, and both groups had the same resistance during adenosine infusion (NT: 0.49 +/- 0.20, T: 0.44 +/- 0.08). In addition, there were no differences in coronary A-V oxygen difference or coronary sinus saturation after training. The data indicate that little change occurs in the maximum flow capacity of the coronary bed following exercise training, and the trained heart responds to tachycardia with a reduced vascular resistance and increase coronary flow in a fashion similar to untrained animals.