During the microvascular operation, vasospasm can be disastrous because it may cause necrosis of the flap. But we could not fully understand the pathophysiology of contraction of operated vessels. We performed microvascular anastomosis of the femoral artery in the rats and decided the maximum initia tension and intensity of electrical stimulus. At the site of operation, postoperative 1st, 2nd, 3rd, 5th and 7th days, we took the samples of vessels including microvascular anastomotic site. Using myometry, we observed the responses of the samples responding to the stimuli and compared the results to that of the other unoperated femoral artery. The contractibility of the artery was increased in the first day and remained continuously until the postoperative 7th days.