The effects of malnutrition and hyperalimentation on wound healing were studied in rats. Progressive weight loss occurred in rats given a protein-free diet, and there was a significant reduction in the mechanical strength of sutured skin and abdominal wounds in rats starved of protein for seven weeks. There was also a significant, but less pronounced, reduction in the tensile strength of colonic anastomoses in severely malnourished rats. Malnourished rats given oral supplements of amino acids for seven days before and after operation had a consistently positive nitrogen balance, and these rats had a significantly higher daily caloric intake than did untreated and normal rats. Amino acid therapy was associated with a significant improvement in the tensile strength and collagen content of abdominal wounds, but it had no measurable effect on the healing of skin wounds or colonic anastomoses. The results suggest that visceral and parietal tissues do not respond in a uniform manner to malnutrition or hyperalimentation, and further studies are required to determine whether or not hyperalimentation has a useful role in the enhancement of wound healing in malnourished subjects.