Animal studies have demonstrated that aprotinin reduces collagen breakdown and improves the healing period of experimental colonic anastomoses. Our objectives in this study were to determine how much the healing period of experimental colonic anastomoses improves in the presence of aprotinin and to see how the xanthine oxidase activity which leads to production of oxygen radicals changes with good oxygenation during the same period. For this purpose, Wistar albino rats were applied to standard colonic anastomoses ten rats were given intravenous aprotinin (Trasylol) for 3 and 7 days. They were sacrificed at the end of these periods. Segment that are including anastomotic line were rejected. Hydroxyproline levels and xanthine oxidase activity were determined by the methods of kivirikko and Hashimoto respectively. The increase in xanthine oxidase activity was found to be significant in the presence of aprotinin (p<0.001).