The effect of dextran 70 and dextran 40 on the rate of thrombosis after end-to-end anastomosis of arteries 3 mm or less in external diameter was studied in dogs. In the group treated with dextran 70 reduction in the rate of thrombosis in vessels with diameters of 2.5 to 3 mm is not significant. In the group treated with dextran 40, however, this reduction is of possible statistical significance. In vessels whose diameters were less than 2.5 mm, treatment with both dextrans reduced the rate of thrombosis significantly. Dextrans, particularly dextran 40, as adjunctive measure in surgical reconstruction of small caliber arteries are strongly recommended for clinical use.