Nitric oxide (NO) was first recognized as the endothelial-derived relaxing factor in 1987, and since then has been extensively studied. It was found to be involved in almost every physiologic or pathophysiologic process in human biology. This study examined the correlation between peripheral blood levels of NO and the microvascular anastomotic patency of an infrarenal aortic anastomosis in a rat model. Four groups of six Sprague-Dawley rats each were studied. Three groups underwent microsurgical manipulation which included: anastomosis followed by ligation of the aorta in one group; anastomosis of the aorta in another; and dissection of the infrarenal aorta in the third group. A fourth group served as controls. Peripheral blood samples for NO levels were taken at different times before and during the first 24 hr after surgery. Results demonstrated a transient rise of NO levels in all operated groups. The ligated group exhibited the most prominent rise in NO blood levels. This rise was statistically significant, compared to the rise in other groups during the first 6 hr postoperatively. The anastomosed group levels were only mildly higher than the dissected group, showing no statistically significant difference postoperatively. NO blood levels in all of the groups returned to baseline at 24 hr postoperatively. The study shows that changes in NO levels in peripheral blood are detectable after microsurgical manipulation of the infrarenal aorta in a rat model. Furthermore, in rats undergoing ligation of the anastomosed aorta, mimicking anastomotic occlusion, NO peripheral blood levels are significantly higher than when the anastomosis remains patent.