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Newey & Smyth (1960a) have shown that dipeptides present in the lumen of the intestine can enter the mucosal cells and undergo intracellular hydrolysis. The present work was undertaken to investigate these processes further. One problem was to determine whether entry of peptide could be accounted for by diffusion, or whether other processes were involved. If entry involves other processes, the question is raised as to the nature of these, and in particular their relationship to the mechanism for entry of amino acids. Having entered the cells the peptide is hydrolysed, and a further question arises about the mechanism of transfer of the amino acids liberated.While entry and hydrolysis of peptide can be shown to take place experimentally, it is not known how far these represent physiological processes. Quite apart, however, from the question whether peptide absorption plays a significant role in absorption of protein, the use of peptides has value from another point of view, in that it provides a means of getting amino acids inside the epithelial cells without these amino acids having entered as such. This offers a possibility of studying cellular mechanisms for amino-acid transfer, and of trying to separate different stages in aminoacid transfer by the cells. Preliminary accounts of this work have been given by Newey & Smyth (1960b. METHODSIn viro. The preparation used was the sac of everted small intestine of the rat (Wilson & Wiseman, 1954a) and the procedure was that described by Parsons, Smyth & Taylor (1958), except that in most of the experiments the sacs were not weighed as fluid transfer was not studied. In the experiments where fluid transfer was studied the sacs were weighed and transfers calculated as described by Parsons et al. (1958). The sacs were made from the middle fifth of the combined jejunum and ileum of the rat. They were filled with bicarbonate saline (Krebs & Henseleit, 1932) with or without added glucose, and were shaken in the same solution with a gas phase of 5 % CO2 and 95 % 02. In anaerobic experiments the gas phase was 5 % CO2 and 95 % N2. Amino acids or peptides were present as described subsequently. At the end of the experiment, which, unless otherwise stated, lasted 30 min the amino acid or peptide present in various sites was determined. In describing the results the * J. G. Graves Research Fellow.

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SUMMARY1. Transfer of sugars, amino acids and fluid and metabolism of glucose were studied with everted sacs of small intestine prepared from fed and 3-day fasted rats.2. In the absence of glucose there was some evidence for increased intestinal transfer of sugars and amino acids in fasted animals. In the presence of glucose there was in general decrease in transfer of amino acids and fluid.3. In fasted animals glucose transfer was reduced except in the lower ileum, and there was a general reduction in glucose metabolism.4. Because of the large reduction in gut weight in fasted animals, expressing transfer on a weight basis is considered not to be a valid procedure in studying the effects of fasting on intestinal transfer.5. The results have been discussed in relation to effects of fasting on energy availability, efficiency of transfer mechanisms, permeability of the intestine and the value of in vitro methods in the study of physiological absorption.