We examined the importance of decreased heart rate in the beneficial effect ofβ-adrenergic blockade on exercise-induced regional myocardial ischemia and contractile dysfunction in conscious dogs with single vessel coronary stenosis (ameroid constrictor). Studies were performed during control treadmill exercise, which produced regional myocardial ischemia (blood flow measured with microspheres) and wall dysfunction (measured using sonomicrometers). A second run was performed after the administration of atenolol (0.3–1.0 mg/kg i.v.), and the reduced heart rate caused by atenolol during early steady-state running was then prevented by atrial pacing during the latter portion of the run. Atenolol reduced the exercise heart rate from 217 ± 25 beats per minute (SD,n= 9) to 166 ± 15, and ischemic zone wall thickening during systole improved from 27 ± 22% of the resting value in the control run to 50 ± 25% of the resting value in the atenolol run (p<0.01). Atrial pacing then increased heart rate to 217 ± 23 beats per minute, and regional wall thickening deteriorated to 15 ± 25% of the resting value. Regional subendocardial blood flow in the ischemic zone during atrial pacing with atenolol was slightly less than that observed in the control run, in both ischemic and control zones, indicating no remaining beneficial effect of atenolol when heart rate reduction was eliminated. We conclude that the only significant mechanism for the improvement in exercise-induced ischemia and wall motion produced by atenolol is a reduction in the exercise heart rate. Furthermore, when the heart rate decrease was prevented, regional blood flow and function tended to be more depressed after atenolol than during the control run.