A method for the production of experimental coronary thrombosis in dogs by the injection of thrombin into the anterior descending branch of the left coronary artery is presented. The clot-dissolving properties of trypsin in the form of Tryptar and Enzar were evaluated. The effect of trypsin was also studied in relation to changes in the clotting mechanism of the blood for the purpose of determining tolerated doses. Regardless of amounts of enzyme or time of administration, there was no evidence to indicate prevention of myocardial infarction.