We examined the effect of age on the adaptive capacity of small bowel mucosa following 60% enterectomy. Two groups of male Fischer 344 rats (3 mo old and 26 mo) underwent either a mid-small bowel transection and reanastomosis (control) or 60% proximal enterectomy beginning at the duodenojejunal junction. Rats were sacrificed at 5, 14, and 21 days after operation, and the mucosa was weighed and assayed for DNA, RNA, protein, and polyamine concentration and content. Ornithine decarboxylase activity was also measured in ileal mucosa at 5 days after surgery. Young rats had completed the adaptive hyperplastic response within 2 weeks after operation by all biochemical measurements; similar adaptation was not seen until 3 weeks after operation in the old rats. We conclude that although the capacity to achieve intestinal adaptation after enterectomy is preserved into old age in rats, this compensatory response is delayed.