The effect of topical heparin application and vascular freezing on thrombosis was evaluated in the femoral vein of rabbits. The femoral veins were crushed, incised transversely and treated by 4 different ways; 1) group 1, heparin treated only, 2) group 2, frozen only, 3) group 3, heparin treated and frozen, 4) group 4, treated with saline as control. The patency was evaluated by empty-and-refill test and thrombus formation was judged by stereoscopic and SEM observation. The results obtained were as follows: 1. Thirty minutes after suture, the patency was more improved in the three groups than the control, but there was no significant difference among groups. 2. Three days after suture, the patency was more improved in the three groups than the control, and was prominent in group 1 and group 3 (P<0.05). 3. Significantly reduced thrombus could be seen in most case of group 3. These results suggest that topical application of heparin and vascular freezing is effective in reducing thrombus formation in microsurgery of vein.