The antithrombotic activity of the tripeptide thrombin inhibitor, D-methyl-phenylalanyl-prolyl-arginal (GYKI-14766), was compared to heparin in a model of canine coronary artery thrombosis. Thrombogenesis was initiated by electrolytic injury of the intimal surface of the left circumflex coronary artery. Drug administration was started 15 min before initiation of intimal injury. Clotting times and ex vivo platelet aggregation were determined on citrated blood samples. Gingival template bleeding times were determined. Clotting times (thrombin time; activated partial thromboplastin time, APTT; prothrombin time, PT) increased in a dose-dependent manner with both anticoagulants. The two anticoagulants selectively inhibited thrombin-induced platelet aggregation. GYKI-14766 and heparin were found to delay thrombosis significantly when compared to vehicle-treated animals; minimum effective antithrombotic doses were 0.25 mg/kg/h and 80 U/kg + 30 U/kg/h, respectively. GYKI-14766 (0.25 mg/kg/h) had no effect on template bleeding time, APTT or PT. Heparin (80 U/kg + 30 U/kg/h), however, was associated with a 2.5- to 3.0-min increase in template bleeding time, a 1.8-fold and 1.7-fold increase in APTT and PT, respectively. Antithrombotic efficacy was achieved at doses of GYKI-14766 that did not affect APTT, PT or template bleeding time, whereas antithrombotic efficacy observed with heparin was associated with significant increases in APTT, PT and template bleeding time. These data demonstrate that the tripeptide thrombin inhibitor, GYKI-14766, could potentially prove to be a safer and more effective antithrombotic agent than heparin.