In two separate blind, randomized studies, 48 rabbits were divided into five groups. Three treated groups received dextran 70 (0.51 g/kg) as a single injection, dextran 70 combined with low molecular-weight heparin (LMWH) (560 IU/kg as anti-FXa and 140 IU/kg as APTT in 3 hr), and dextran 70 plus tranexamic acid (14 mg/kg) following severe arterial trauma (arteriotomy/intimectomy). Two control groups received the same amount of saline. The bleeding times from the arteriotomy were recorded, and patency rates and the weight of thrombotic materials were registered 2 hr after reperfusion of the trauma region. The bleeding times in three treated groups, all including dextran, were significantly prolonged compared to the control groups (P < 0.05 or 0.01). The patency rates of treated groups, which were 100% (18/18 vessels patent) in the dextran-treated group, 90% (18/20 vessels patent) in the dextran+LMWH group and 95% 19/20 vessels patent) in dextran+tranexamic acid group, were significantly higher than those of their control groups (55-67% patent vessels) (p < 0.05 or 0.01). The mean weights of thrombotic materials were significantly reduced in the treated groups compared to the corresponding control groups (p < 0.01). In conclusion, dextran 70 in an ordinary dose exerted such a profound antithrombotic effect (100% patency) in small traumatized arteries that the addition of a high dose of LMWH could not further improve patency rates or decrease thrombotic materials but did not prolong vessel bleeding times compared to the single dextran treatment.(ABSTRACT TRUNCATED AT 250 WORDS)