Feeding intolerance is frequently reflected in preterm infants by delayed gastric emptying. Gastric emptying is delayed by the physical characteristics of ingested nutrient as well as the rate of feeding. Because gastric emptying is dependent upon duodenal function, the present studies were undertaken to assess duodenal motor responses to feeding of differing nutrient content and rate of feeding. Using a Latin square design we recorded duodenal motor responses in 14 preterm infants given four test feedings in random order over 18 h. Three were given as a 120-min infusion containing no nutrient, a 10 cal/oz formula, and a 20 cal/oz formula. The fourth test feeding consisted of a 20 cal/oz formula given as a bolus over 15 min. Although caloric density was altered, osmotic load and nutrient proportions of the formulas were not. Motor responses were recorded using a low compliance continuous perfusion manometric system. When infants were fed "water" and half-strength formula as a slow infusion, they demonstrated little or no duodenal motor response to feeding. When these infants were fed full-strength formula as a slow infusion, they displayed a brisk increase in motor activity (p < 0.05), but profound motor quiescence when fed the same volume by bolus over 15 min (p < 0.05). Of the four test feedings, only full-strength formula given as a slow infusion triggered adult-like duodenal motor responses to feeding. We speculate that feedings of full-strength formula given slowly by infusion will improve feeding tolerance.