The effect of a low-protein diet on the healing of colonic anastomoses in the rat was studied. The low-protein diet given for 2 weeks caused a weight loss of 11.9%. The healing of colonic anastomoses was assessed by bursting pressure measurement performed sequentially in the postoperative period. Compared to control animals, rats on a low-protein diet had significantly lower bursting pressures from the 4th to the 7th postoperative day. There was no difference in the hydroxyproline concentration at the anastomoses of the two groups of rats. These results suggest that the 5th postoperative day is the most appropriate time to investigate the effects of protein malnutrition on colonic healing in the rat.