These experiments assessed the ability of the neutral detergent fiber (NDF) method to remove starch and nitrogen from food NDF residues, and nitrogen and endogenous secretions from fecal NDF residues; and compared indices of bowel function, including apparent NDF digestibility, of nine elderly subjects to those of six young women. Subjects consumed similar, controlled 'low fibre' diets containing foods typically eaten in the United States. Nitrogen and starch contents of food composite NDF were determined in NDF obtained by the hog α amylase modification of the Van Soest procedure, whereas the nitrogen content of fecal NDF was determined using NDF obtained by the unmodified procedure. The modified Van Soest method adequately removed nitrogen and starch from food NDF, but fecal NDF contained nitrogen. Both groups digested the majority of the NDF, 63 and 70% for the elderly and young subjects, respectively. Apparent fiber digestibility, which was measured several times in each subject, increased to 69 and 75% for the two groups, respectively, when a correction was made for the nitrogen in the fecal NDF. Mean gastrointestinal transit time and frequency of defacation of the two age groups were comparable, whereas mean daily wet and dry stool weights of the elderly were larger than those of the young women. These results suggest that the bowel of the elderly may function as well as that of the young adult.