Thirty consecutive patients (19-71 years of age) undergoing abdominal operations and having feeding catheter jejunostomy tubes placed, were prospectively randomized and studied for tolerance to elemental (amino acid) or polymeric (intact protein isolates) dietary formulas. The groups were matched for sex, age, height, weight, operations, and caloric goal. Nutritional profiles of the patients were similar preoperatively. The number of stools/day and stool consistency were direct indicators of tolerance; nutrient intake was an indirect indicator. Diarrhea is defined as greater than or equal to five stools/day. We found no significant difference between the groups. There were no patients with diarrhea on days 1 or 2. Formula intake as an indirect measure of tolerance was not significantly different for the two groups from days 1-4. On day 5, intake of the elemental group exceeded that of the polymeric group (p less than 0.02). Enteral caloric intake as percent of caloric goal did not differ. Nitrogen intake, urinary nitrogen, serum prealbumin, body weight, serum sodium, and serum potassium did not differ significantly between the two groups throughout the study. Serum chloride decreased significantly (105 to 99.8 mEq/L) in the elemental group (p less than 0.03). Our study shows that patients undergoing routine gastrointestinal operations will tolerate early enteral feedings (day 1) and that there is no significant difference in tolerance to elemental or polymeric dietary formulas.