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The relation between the rate of absorption of a substance from the intestine and its concentration has frequently been used as a criterion of the process involved in absorption. Hober & Hober (1937), Lathe (1943) and others have concluded that the rate of absorption of amino acids is not proportional to the concentration in the intestine, and regarded this as evidence of the presence of an active process concerned with absorption. Hetenyi & Winter (1952) found that the rate of absorption of glycine, histidine and ,B-alanine was not proportional to concentration, while in the case of proline there was a straightline relationship. The following experiments were undertaken to study more carefully the relationship between the rate of absorption and concentration of amino acids, and furthermore the D-and L-enantiomorphs of different amino acids were compared. A preliminary account has been given by Jervis & Smyth (1959b). METHODSAlbino rats were anaesthetized with pentobarbitone sodium and the procedure for study of absorption was that described by Jervis, Johnson, Sheff & Smyth (1956). By this technique it is possible to measure both the rate of absorption and also the concentration of the substance in the lumen of the intestine. The amino acids, neutralized by addition of NaOH, were dissolved in NaCl solution 09 % (w/v) and circulated through the lumen of the intestine for 20 min. At the end of this period the amount remaining in the intestine was washed out and estimated, and from this and that initially added the amount absorbed was calculated.Chemicals. The amino acids used were D-and L-methionine and histidine and were obtained commercially.

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