The osmolality of gastric and duodenal contents were determined simultaneously during 30 test-meals at 0, 45, 90, 135 and 180 min after feeding in 15 low birth weight infants, birthweight: 2075 +/- 330 g. gestational age: 35.4 +/- 1.8 weeks and postnatal age: 8 +/- 4 days. These infants were fed human milk or various formulae whose osmolalities ranged from 227 to 622 mosmol/kg. With human milk and isotonic formula, the osmolality in the stomach and duodenum remained close to 295 mosmol/kg throughout the test. With hypotonic formula (227), the osmolality in the stomach and duodenum rose during the first 45 min to a plateau of about 266 and 285 mosmol/kg, respectively. With the two most hypertonic diets (about 600) (10 g/dl glucose solution and an elemental formula), the osmolality in the stomach and duodenum remained high throughout the test, 470 +/- 73 mosmol/kg at 45 min and 345 +/- 55 mosmol/kg at 180 min. There was a significant positive linear correlation between the osmolality of the diet and the osmolality in the stomach and duodenum at each time of sampling but the slope of the regression lines decreased progressively during the 3 hours after feeding. On the pooled data, there was also a significant positive linear relationship between duodenal and gastric osmolalities. This study suggests that hyperosmolar feedings might be dangerous for the preterm infants and should be avoided if possible.