During a period of semistarvation there is a fall in the mass of body cells and a lowering of the basal metabolic rate (Wishart, 1934;Taylor & Keys, 1950;Grande, Anderson & Keys, 1958), the degree to which each change occurs depending upon the duration of the experiment and the severity of the dietary restriction. The loss of cellular mass and the fall in the metabolic rate depend upon the number of calories available, and provided that the caloric restriction is not too severe an equilibrium situation can be obtained. It was of interest therefore to investigate the absorptive ability of the small intestine during semistarvation, as it seemed a vulnerable organ in view of its great demands on the metabolic pool for the frequent replacement ofits epithelial lining (Leblond & Stevens, 1948;McMinn, 1954).We have found that after a period of semistarvation sufficient to cause a loss of about 20 % of the initial body weight, there is an enhanced rate of disappearance of both glucose and L-histidine from the small intestine of the rat in vivo, and that in vitro the small intestine from semistarved animals can transport both these substances against a concentration gradient to a greater extent than can the small intestine of rats fed on an 'ad libitum' diet. A preliminary report has been given by Neame & Wiseman (1959). METHODS Animal8 and diet. Male albino rats of an inbred strain were used, and were kept individually in separating cageswith free access to water. The food used throughout was Diet 86, purchased from The North-Eastern Agricultural Cooperative Society, Ltd., Bannermill Place, Aberdeen, its composition being: soluble carbohydrate 53.4%; protein, 20-0 %; fat, 3.8%; fibre, 3-3%; ash, 5-2%; moisture, 14-3%.All animals were inspected every day and the dietary regimen was as follows: Group A was fed ad libitum. Group B was fed 5 g food per rat per day for 5 days. Group C was fed 5 g food per rat per day for 9 days. Group D was fed 5 g food per rat per day for 9 days, and then fed ad libitum for 24 hr. Group E was fed 5 g food per rat per day for 9 days, and then fed ad libitum for 3 days. Group F was fed 5 g food per rat per day for 9 days, and then fed ad libitum for 7 days.

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The fate of ingested protein was re-investigated by Dent & Schilling in 1949, and they concluded that heterologous protein is absorbed almost entirely as free amino acids and that the concentration of the amino acids in the portal blood after eating casein is largely what one would expect from a knowledge of the amino acid content of that protein. Glutamic acid, however, is exceptional in that its concentration in the portal blood is much lower than would be expected in view of the high glutamic acid content of casein. In 1953 Wiseman investigated the ability of the isolated small intestine of the rat to transport various amino acids against a concentration gradient, and found that although the L-forms of alanine, phenylalanine, methionine, histidine and isoleucine are absorbed against a concentration gradient, those of glutamic and aspartic acids are not. He also found that part of the glutamic and aspartic acids disappears from the experimental system as a whole. Matthews & Wiseman (1953), using an in vitro technique (Wiseman, 1953), then showed that when either glutamic acid or aspartic acid is introduced into the intestinal lumen, alanine appears in the fluid bathing the serosal surface of the intestine. They concluded that transamination occurs during the absorption of glutamic or aspartic acid and results in the appearance of alanine in the portal blood. The following experiments, investigating this process in vivo, confirm their hypothesis, and also give the quantitative relationship between these amino acids during absorption from the dog's small intestine. METHODSExperimental procedure. Dogs, weighing between 12 and 20 kg, were used and were allowed only water during the 24 hr before the experiment. Intravenous pentobarbitone (approximately 30 mg/kg body weight), intraperitoneal hexobarbitone (approximately 50 mg/kg body weight), or ether was used for induction of anaesthesia, and then surgical anaesthesia was maintained by intravenous pentobarbitone. A femoral vein was exposed and used for any intravenous injections required (pentobarbitone and heparin). A carotid artery was exposed and cannulated with a T-shaped glass cannula and the arterial blood pressure recorded from time to time. The side arm

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