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Entry rates of glucose in sheep have been estimated by Annison & White (1961, 1962) by using isotope-dilution techniques. The values obtained were surprisingly large in view of the small absorption-of carbohydrate from the alimentary tract of sheep (Annison & Lewis, 1959) and the limited amount of .propionate available for conversion into glucose (Annison, Hill & Lewis,, 1957; Annison & Lindsay, 1962). When interpreting these results we have to consider to what extent a turnover rate, as measured by isotope dilution, may be-equated with glucose requirement. Annison & White (1961) drew attention to the fact that an internal cycle such as that described by Cori & Cori (1928) would be included in such an estimate of turnover rate although it would have no significance in relation to carbohydrate requirements. The existence of the Cori cycle (muscle glycogen-+ lactate-* liver glycogen-+ glucose muscle glycogen) was inferred by Cori from experiments in animals under the action of adrenaline. $uch texts as refer to it (Lovatt Evans, 1945) suggest that it is of importance only under the action of adrenaline or exercise. However, Andres, Cader & Zierler (1956), on the basis of arteriovenous difference measurements of blood glucose, lectate, oxygen and C02 in the resting human forearm, suggested that a major fraction of glucose utilized by muscle was dissimilated to lactate. Drury & Wick (1956) used L(+)-[14C]lactate to demonstrate that lactate oxidation at resting concentrations of blood lactate could account for up to 25% of the C0. output of rabbits. These results suggest that the Cori cycle could be more important than is commonly supposed, even under resting conditions. There appears to be no pub. lished evidence relating to ruminants. A constant infusion of uniformly labelled

This investigation was made possible by the generous support of the Australian Wool Research Committee. Mr J. Roberts and Mr J. Wilcher gave valuable technical assistance.

glucose and free fatty acids in the whole animal, but the use of isotopically labelled substrates has allowed some parameters to be defined. SUMMARY 1. The rates of entry of glucose and acetate in fed and starved sheep were measured simultaneously by the constant infusion of labelled glucose and labelled acetate. 2. The entry of acetate after feeding was sufficiently constant to allow measurement of entry rates only during the period when a maximum concentration of rumen acetate was attained, which presumably coincided with maximum production and absorption of acetate. 3. The entry of endogenous acetate in sheep with emptied rumens was 40-50 % of the value for the total entry of acetate obtained in the same animals after feeding. Under conditions of normal acetate absorption, the entry of endogenous acetate accounted for about 25 % of the total entry. 4. Raised concentrations of blood glucose or acetate reduced the entry of endogenous acetate in sheep with emptied rumens, or in starved sheep. These results were consistent with the hypothesis that the oxidation of free fatty acids contributes substantially to the entry of endogenous acetate in sheep. This investigation was made possible by the generous support of the Australian Wool Research Committee. Mr J. Roberts and Mrs J. Musgrave gave valuable technical assistance.

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