Anastomotic healing after obstruction of the small intestine was investigated by measurements of development of strength, content of collagen and synthesis of collagen in standardized specimens taken at biopsy of the intestinal wall on both sides of the anastomosis. One hundred and sixteen male Wistar rats were used. Forty-eight rats were used for studies of synthesis of collagen by in vivo labeling with 100 microcuries of tritiated L-proline. Anastomoses performed after two days of obstruction in the ileum had a higher rate of complications than operations performed upon control rats (p less than 0.05). Obstruction of the small intestine resulted in increased content of collagen of the ileal wall as a consequence of increased synthesis of collagen, but this resulted in neither increased capacity to hold suture nor in any differences in development of strength between rats operated upon for obstruction of the small intestine and control rats. Increased turnover of collagen was noted in the vicinity of the anastomoses after obstruction, which might be related to the increased rate of complications found. Rats with complications accumulated more collagen in the intestinal wall after one week compared with rats with obstruction and uncomplicated healing. These rats had persistent obstruction or formation of granuloma, or both.