Current therapeutic use of heparin as an adjunct to thrombolytic therapy for myocardial infarction is suboptimal with respect to efficacy and bleeding risk. In a rat carotid arterial thrombolysis model (FeCl3-induced injury) we evaluated the combined effect of tPA (2.0 mg/kg/30 min) with our potent injectable direct thrombin inhibitor, BCH-2763 (Ki 0.11 nM; MW 1.5 kDa), which, unlike heparin, inhibits bound and free thrombin; comparisons were with standard heparin (SH), other direct thrombin inhibitors, r-hirudin (MW 6.5 kDa) and hirulog (MW 2.3 kDa), or tPA alone. Time to lysis (TL), patency time (PT), aPTT (fold increase) and bleeding time (BT) were determined. ED100 (100% of rats reperfused) for BCH-2763, hirulog or r-hirudin was 1, 3 or 2 mg/kg/60 min, respectively; 67% of rats reperfused with SH at the highest dose tested (220 U/kg/60 min) and 43% with tPA alone. At these doses, TL (min) was shorter (p < 0.01) with BCH-2763 (0.5 +/- 0.1), hirulog (3.3 +/- 2.3) or r-hirudin (2.3 +/- 1.0) than SH (66.3 +/- 30.8) or tPA alone (93.4 +/- 21.4). The aPTT fold increase after 15 min infusion was markedly greater (p < 0.001) for SH (32.0 +/- 0.8) than BCH-2763 (3.7 +/- 0.5), hirulog (5.2 +/- 0.3) or r-hirudin (4.5 +/- 0.8) in combination with tPA or tPA alone (1.1 +/- 0.1). In addition, the BT (min) for BCH-2763 (3.0 +/- 0.4) was similar to tPA alone (1.6 +/- 0.3), but prolonged (p < 0.05) for hirulog (7.5 +/- 2.7), r-hirudin (6.6 +/- 0.8) or SH (7.3 +/- 1.8). Comparisons at same aPTT fold increase revealed that in combination with tPA, BCH-2763 required a lower anticoagulant level to shorten the TL and prolong the PT than hirulog, r-hirudin or SH. Thus, in this rat arterial thrombolysis model direct thrombin inhibitors are more effective than SH as antithrombotic adjuncts to tPA. BCH-2763 is effective at a lower gravimetric dose and more modest aPTT fold increase than hirulog or r-hirudin with less alteration in haemostasis, which may confer an improved safety index.