It has been suggested that colonic healing is impaired in malnourished subjects, but there have been no biochemical studies of the effect of malnutrition on colonic healing. The effects of malnutrition on the colon and the healing of colonic anastomoses were studied in rats fed a protein-free diet. Test animals were compared with control animals of similar age, and control animals of similar weight. There was a significant reduction in the body weight, total serum proteins and serum albumin of animals starved of protein. Malnutrition resulted in a reduction in the weight of the uninjured colon, and an increase in the colonic collagen concentration. There was a significant reduction in the collagen content of the colon in animals starved of protein for seven weeks, and the collagen content of anastomoses in these animals was significantly lower than the value in control animals. Anastomotic edema occurred during colon healing in animals starved for seven weeks. Measurements of colonic bursting pressure were an inaccurate guide to colonic healing. It is concluded that severe malnutrition resulting in 34% loss of body weight had an adverse effect on colonic healing.