I . The rates of production, or entry rates, of acetic, propionic and butyric acids in the rumen of grazing sheep have been measured by a radioisotope technique previously used on penned animals. 2. Regression equations were derived relating entry rates to ruminal concentrations. The equations from penned and grazing sheep did not differ significantly. 3. The standard errors of entry rates predicted for a single sheep from equations based on all results were calculated to be k0.60, 0.18 and

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... m-mole/min for acetic, propionic and butyric acids respectively. Interconversions between acetic and butyric acids in the rumen were similar in extent to those found in penned sheep. These values were used to calculate from entry rates the net amounts of the acids becoming available to the animals. 5. An equation was derived that permits the amount of energy supplied to an animal by the acids to be predicted from concentrations with a standard error of f 0.17 kcal/min. 6. The applicability of the prediction equations is discussed.