The distribution of labelled autologous platelets in the myocardium was studied in open-chest dogs following a nonthrombotic coronary artery occlusion. A significant accumulation of platelets occurred in the ischaemic and borderline ischaemic myocardium both 15 and 150 min after coronary artery occlusion, most pronounced after 150 min. The small platelet accumulation after 15-min ischaemia might be due to stasis of blood since a similar increase in labelled erythrocytes in that area was observed. The platelet trapping after 150 min most likely represents platelet aggregates. In spite of the marked increase in platelet trapping, the myocardial blood flow in the ischaemic area increased significantly from 15 to 150 min of myocardial ischaemia. It is concluded that the increased platelet trapping in the first hours following a nonthrombotic coronary artery occlusion was insufficient to impair coronary circulation.