Dogs were given dipyridamole, 4 mg. per kilogram three times daily for three months, and were then compared with controls, following myocardial infarction caused by anterior descending artery ligation. Collateral blood supply was assessed by measurement of radioxenon clearance from the infarct every half hour for 2 hours. In the treated group, there was a 29 per cent increase in clearance (p < 0.1) and a 50 per cent increase in collateral capacity (p < 0.01) expressed as the ratio of clearance to perfusion pressure. © 1969.