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It has been known for a long time that when fatty acids are incubated with liver or kidney slices acetoacetate is formed. In the last few years ketone body production from fatty acids in the presence of cell-free extracts of these organs has been intensively studied (see Lehninger, 1952). Recently it has been shown that, when short chain saturated fatty acids are incubated with sheep rumen tissue, ketone bodies accumulate in the medium at the expense of the acids (Pennington, 1952). In view of similarities of function of guinea-pig caecum and sheep rumen (Hagen and Robinson, 1953), we thought it would be interesting to observe whether ketone body production took place when guinea-pig caecal tissue was incubated with short chain fatty acids.This communication reports a study of the ketone body production and oxj'gen consumption when guinea-pig caecal tissue is incubated with small quantities of the sodium salts of acetic, propionic, butyric, valeric, caproie and heptoic acids. METHODS.Guinea-pigs were killed by a blow on the head. Their caeca were immediately removed and slit at either end so that the contents could be washed out with ice-cold 0-9 p.c. (w/v) sodium chloride solution. The caeca were then placed in ice-cold saline solution and cut into several pieces, which were washed and rewashed until the washings were quite clear. Finally, the tissue was chopped into fine pieces with scissors; these were dried between filter paper and 100 mg. placed into Warburg flasks, each containing 3 ml. of Krebs-Ringer phosphate buffer and having KOH and filter paper in the centre well. The fatty acid as the sodium salt at pH 7 was placed in the side arm of the flask. The flasks were shaken in a bath at 37° C. Eespiration was usually allowed to proceed for an hour before tipping in the acid. This procedure was adopted in the first experiments when it was thought that a quantitative correlation might be made between oxygen uptake and acetoacetate production and that the initial oxygen consumption without substrate might serve as a measure of actively metabolizing tissue in each flask; later the procedure was continued in order that subsequent experiments might be compared with earlier ones. Shaking was continued for n further two hours after adding the substrate and oxygen consumption was measured at 15-minute intervals throughout this period.After incubation, the flask contents were emptied into centrifuge tubes, each flask washed with 3 ml. water and the washings added to the tubes. After centrifugation, 4 ml. of the supernatant fluid were used for ketone body determination by the method of Greenberg and Austral.

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