An evaluation was made of the effects of increasing age on the mechanical properties and the collagen content of healing wounds in stomach and duodenum of rats. Wounds were made in the non-glandular (rumen) and the glandular oxyntic part (corpus)of the stomach and in the duodenum of rats that were 26 months old. The wounds were tested 7 and 20 days after the operation. The results of these tests were compared with those made on a group of young rats (3-4 months old) that were subjected similar wounds. Aging increased the stiffness and decreased the extensibility of tissue taken from intact stomach (rumen and corpus) and from wounded rumen. No differences in the stiffness and extensibility of the tissue from the duodenum wounds could be demonstrated. Except for a higher breaking strength in samples of rumen from old animals after 20 days of healing, no differences in mechanical strength measurements (breaking strength and breaking energy) from intact and wounded tissue could be demonstrated in the old and young animals. The collagen content of intact as well as wounded stomach tissues appeared related to age. No differences in the collagen content in the tissue from the duodenal wounds wee found. The dimensions of the biochemically active zone around the incision remained essentially unchanged in the duodenum and stomach and it was of the same width as that found for young animals. This indicates that aging has no adverse influence on wound healing in the stomach and the duodenum of rats.