There is an absence of data on the timing of occlusion of vessels after anastomosis, and on the possible subsequent reopening (recanalization) of these vessels. This lack of information may be an important factor in the wide discrepancies found among reported patency rates for laboratory microvascular repair. In this study, a total of 300 standard microsurgical anastomoses were performed on rat femoral veins. The patency of each anastomosis was assessed at regular intervals within a 2-week study period. These results showed that the majority of venous occlusions occurred within 1 day after repair. Recanalization of the occluded vein was first seen at day 3 postoperatively. Recanalization was observed over a 2-week postoperative period with increasing frequency. The authors conclude that the optimal time to assess the technical outcome of experimental venous patency is 1 to 2 days after the repair.