Fat and nitrogen absorption and fecal characteristics were studied in 10 healthy, young adults who ingested a free-choice diet for 12 days and then isocaloric amounts of one of two nutritionally complete liquid defined formula diets during a succeeding 12-day period. Apparent fat absorption was 98 to 99% of intake with both defined formula diets and 95% of intake on the free-choice diet. Nitrogen digestibility was determined during the liquid diet period, and averaged 95 to 98% of intake. There were no statistically significant differences between the liquid diets and the free-choice diets with respect to fecal wet or dry weight, transit time, or stool frequency. Individual stool weights averaged 83 to 96 g on the free-choice or liquid formula diets and were comparable to other values reported in the literature. These results support the theory that a critical volume of feces must accumulate in the colon before defecation occurs, regardless of diet composition.