Animals exercised strenuously during two days after left anterior descending coronary artery occlusion, the first session being within one hour, showed no adverse effects compared with a control group. There was no increase in size of infarction, cardiac rupture, ventricular aneurysm, or mortality. Though peak heart rates exceeded 220/min, and in some animals rates as high as 300/min were reached, only two dogs showed major arrhythmias while exercising. This type of animal model may provide significant insights into the effects of exercise on the ischaemic myocardium.