It is well known that large intestine anastomoses tend to leak much more frequently than do small intestine anastomoses. These studies were undertaken to elucidate the pathophysioplogic basis of this phenomenom. Single layer interrupted silk suture anastomoses were performed in both the distal part of the ileum and the distal part of the colon in 29 dogs. The anastomoses were resected at one through eight days and 19 to 31 days after operations. Collagen response, vascular response and bursting strength at the site of anastomoses were investigated. The results demonstrated a statistically significant decrease in the collagen and vascular response of the large versus the small intestine. This differential response may play a role in the mechanism of the relatively high incidence of suture line separations observed after colonic anastomoses.