1. The production rate of propionate in the rumen and the entry rate of glucose into the body pool of glucose in sheep were measured by isotope-dilution methods. Propionate production rates were measured by using a continuous infusion of specifically labelled [(14)C]propionate. Glucose entry rates were estimated by using either a primed infusion or a continuous infusion of [U-(14)C]glucose. 2. The specific radioactivity of plasma glucose was constant between 4 and 9hr. after the commencement of intravenous infusion of [U-(14)C]glucose and between 1 and 3hr. when a primed infusion was used. 3. Infusion of [(14)C]propionate intraruminally resulted in a fairly constant specific radioactivity of rumen propionate between about 4 and 9hr. and of plasma glucose between 6 and 9hr. after the commencement of the infusion. Comparison of the mean specific radioactivities of glucose and propionate during these periods allowed estimates to be made of the contribution of propionate to glucose synthesis. 4. Comparisons of the specific radioactivities of plasma glucose and rumen propionate during intraruminal infusions of one of [1-(14)C]-, [2-(14)C]-, [3-(14)C]- and [U-(14)C]-propionate indicated considerable exchange of C-1 of propionate on conversion into glucose. The incorporation of C-2 and C-3 of propionate into glucose and lactate indicated that 54% of both the glucose and lactate synthesized arose from propionate carbon. 5. No differences were found for glucose entry rates measured either by a primed infusion or by a continuous infusion. The mean entry rate (+/-s.e.m.) of glucose estimated by using a continuous infusion into sheep was 0.33+/-0.03 (4) m-mole/min. and by using a primed infusion was 0.32+/-0.01 (4) m-mole/min. The mean propionate production rate was 1.24+/-0.03 (8) m-moles/min. The conversion of propionate into glucose was 0.36 m-mole/min., indicating that 32% of the propionate produced in the rumen is used for glucose synthesis. 6. It was indicated that a considerable amount of the propionate converted into glucose was first converted into lactate.

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I. Glucose entry rates into the blood and propionate production rates in the rumen have been measured in sheep given rations containing varying proportions of starch and roughage (lucerne).2. Glucose entry rates and propionate production rates were similar for all rations studied. 3. The proportion of the glucose entry rate arising from propionate produced in the rumen was highest on the ration containing the greatest quantity of lucerne and decreased as the proportion of starch in the ration increased. Rate of conversion of propionate into glucose was estimated and was found to decrease as the amount of starch in the ration increased.4. Concentrations of total volatile fatty acids (VFA) in the ruminal fluid were lowest in the ration with the greatest proportion of starch, implying lower VFA production rates with the starch rations although intakes of digestible energy were approximately the same. The mean concentrations and rates of production of propionate in ruminal fluid were similar for all rations. 5 . The low VFA concentrations and the reduced conversion of propionate into glucose on the high rations, despite similar propionate production rates and glucose entry rates, may have been due to starch escaping ruminal fermentation. I t is suggested that this glucose absorption may have reduced gluconeogenesis from propionate.In ruminants, gluconeogenesis is a major biosynthetic process, since apparently only in animals given a high proportion of starch in their diets could there be significant absorption of glucose from the alimentary tract (MacRae & Armstrong, 1966; Lindsay, 1959; Armstrong, 1965). Estimates of glucose entry rates in non-pregnant sheep have varied from 1.4 to 4 3 mg/kg per min (Annison & White, 1961 ;Kronfeld & Simesen, 1961;Bergman, 1963; Ford, 1963 Ford, , 1965Bergman, Roe & Kon, 1966;Leng, Steel & Luick, 1967). The variations in estimates may be in part attributed to differences in the feeding regime and in the techniques adopted in the various laboratories. For instance, in most studies animals were fed once or twice daily and entry rates were estimated some time after the animal had eaten; this usually coincided with maximum volatile fatty acid (VFA) concentration in the rumen. Ford (1965) has shown that glucose entry rate in sheep varied with the quantity and quality of food eaten and he suggested that there was an increase in gluconeogenesis as feed intake increased.In sheep given lucerne at hourly intervals, propionate produced in the rumen contributed 54 % of the carbon of the glucose synthesized (Leng et al. 1967), whereas it was shown by Bergman et al. (1966) that absorbed propionate contributed 27 yo of the glucose entry rate. As only a relatively small proportion of the propionate produced in the rumen was converted into glucose, it appeared that on the diets of lucerne

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