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small formation of ketone bodies which accompanied the metabolism of acetate was sharply depressed by glucose, in spite of the increased uptake of acetate. The ketone-body production from butyrate was affected to a relatively much smaller extent by glucose. DISCUSSIONAlthough the proportions may vary, all three acids used in this study are known to be present in the rumen, on a wide variety of natural diets. It is evident that a reliable quantitative estimate of the importance of the rumen epithelium in metabolizing the fatty acids produced in the rumen cannot be deduced from experiments with the single acids. In the experiments of Table 1, where equal amounts of the acids were present in the mixtures, the most definite effects were the inhibition of propionate uptake by butyrate and of butyrate uptake by acetate. In the normal rumen, with different proportions of the acids, other interrelationships may become more important.Insufficient information is available to attempt to interpret the results. However, it does not seem that they can be explained by simple competition of the fatty acids for a common activating enzyme. The apparent interconversion of acetate and butyrate in the presence of propionate is of particular interest and merits further investigation. The effect of propionate and glucose upon ketone-body production from acetate accords with their known antiketogenic activity. However, the results suggest that, even in the presence of these compounds, butyrate will be an important source of ketone bodies in the sheep. SUMMARY 1. Butyrate lowered the uptake of propionate by sheep-rumen epithelium, and acetate lowered the uptake of butyrate.2. Ketone-body production from acetate was suppressed by propionate or glucose.3. Some acetate was produced when propionate and butyrate were metabolized together.

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