Total entry rate of blood glucose and the rate of irreversible loss of blood acetate and its oxidation have been examined in sheep at rest and while walking on a horizontal treadmill at 5 km/h for 2 h. Sheep were given their daily ration of 1000 g chaff in 24 equal portions at hourly intervals and received multiple intravenous injections of [2-3 H]glucose and intravenous infusions of [1-14 C]acetate and NaH14C03 • At rest the total entry rate of blood glucose was 0·44 ± 0·03 mmol/min (values

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... en as mean ± s.e.m. for four sheep), the glucose pool was 23 ± 1 mmol and the rate of irreversible loss of blood acetate was 2·3 ± 0·1 mmol/min. During exercise, the total entry rate of blood glucose was 0·84 ± O· 04 mmol/min, the glucose pool was 27 ± 2 mmol and the rate of irreversible loss of blood acetate was 2·6 ± O· 1 mmol/min. Gluconeogenesis apparently increased markedly in response to exercise as indicated by the incorporation of 14C from blood bicarbonate into blood glucose. Despite the substantial increase in the rate of irreversible loss of blood bicarbonate (from 11·6 ± 1 to 20·2 ± 2 mmol C/min), and hence energy expenditure with exercise, only a slight change was recorded in the proportion of the irreversible loss rate of acetate that was oxidized.