The effect of the fibrinolysis inhibitor tranexamic acid on early thrombus formation following microvascular trauma was investigated in the central arteries of ears in 86 rabbits (in all 172 vessels), divided into four separate blind randomised studies. In the first part a common end-to-end anastomosis was done and in the last three studies a severe trauma-arteriotomy/intimectomy was performed. Parameters studied were vessel bleeding times, patency rates, weights of intraluminal thrombotic material, haematocrit and plasma fibrinolytic activity. In the first study consisting of 14 control animals and 18 animals treated with 14 mg/kg bw of tranexamic acid, end-to-end anastomosis was performed on the central artery of one ear and on the central vein of the other ear. In the second, third, and fourth studies consisting of 18, 20, and 16 control vessels and the same number of corresponding vessels in treated animals a 7-mm longitudinal arteriotomy followed by a deep 5-mm-long intimectomy was performed. The second and third treated groups were given 14 mg/kg bw of tranexamic acid 5 min and 1 h, respectively, before reflow and the fourth group 28 mg/kg bw 5 min before reflow. The difference between the second and third studies was the addition, to mimic clinical situations, of 8.5 ml saline/kg bw 2 h before reflow in the third study. In conclusion, treatment with a single clinical dose, 14 mg/kg of tranexamic acid, did not influence vessel bleeding times or thrombus formation in the anastomotic or severe trauma models and seems safe to use. Not even a double clinical dose, 28 mg/kg, influenced thrombus formation in a statistically significant way.