||Three normal young adults were intubated with a double-lumen tube according to a previously described method (10). Subjects were fasted for 8h prior to intestinal perfusions. The position of the tube was checked radiographically. When the end of the tube had reached the upper jejunum, 120 cm from the teeth, (~25 cm beyond the ligament of Treitz), perfusions were carried out by a peristaltic pump at a constant rate of 15ml/min, through the proximal opening of one lumen of the polyvinyl tube. Samples were collected by siphonage from the distal opening in the other lumen, 30 cm away from the perfusion site. Sixteen millimoles of the pure L-form of aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, valine, methionine, isoleucine, leucine, phenylaianine, iysine, tryptophan, histidine, and arginine were rinsed into 700 ml of H20, heated to 70°C in a water bath and stirred with magnetic Stirrers until all the amino acids were dissolved. Sixteen miliimoles of cystine and tyrosine were dissolved in 20 ml of 6 N HCl and diluted to 500 ml with water. Serial perfusion of the jejunum of three normal subjects normal subjects with equimolar mixtures of 18 essential L-amino acids. Six separate intestinal collections of three subjects were used to calculate the mean ± 1 SD.