Studies are reported on the volatile fatty acid (V.F.A.) metabolism of sheep and bovines with particular reference to the association between (i) ingestion of food and ruminal levels of V.F.A.s and arterial levels of acetic acid and (ii) the utilization of arterial acetic acid by the bovine mammary gland and the association between this utilization and the proportion of lower fatty acids (Reichert-Meissl value) in the milk fat. Ruminal levels of V.F.A. and arterial levels of acetic acid were found to be similar in cattle and sheep, and similar to those reported by earlier workers for sheep. There was a close association between changes in ruminal V.F.A. and arterial acetic acid levels. Arterial acetic acid levels were found on the feeds studied to reach a maximum value of 8-14 mg. per cent. by 2-5 hours after feeding, declining to 5-8 mg. per cent. by 8 hours after feeding and to 2-6 mg. per cent. by 16 hours after feeding. On starvation for approximately 72 hours, values fell as low as 1.5 mg. per cent. Acetic acid was found to be a major metabolite of the bovine mammary gland, arterio-venous (A-V.) differences being directly dependent on the arterial level and of the order of 2-6 mg. per cent. or 40-80 per cent. of the arterial level in the fed animal. Arterial levels and mammary A-V. differences of acetic acid were unaffected by cod-liver oil feeding or low roughage-high concentrate diets, both of which depressed the fat percentage and the Reichert- Meissl (R-M.) value of the milk fat. Hyperinsulinism and recent or delayed milking also had no effect on the A-V. differences. The depression in R-M. value during fasting was not reversed by intraruminal or intravenous acetic acid infusions despite the maintenance of high blood levels of acetic acid. There was no detectable correlation between carbon dioxide output by the mammary gland and the acetic acid uptake of the gland, indicating that the acid served some 'useful' purpose in the gland. It is concluded, taking into account other evidence, that acetic acid is utilized in the gland for fat synthesis and oxidation, depending on the requirements of the gland, but that the proportion of lower fatty acids in milk fat is not dependent on the uptake of acetic acid.

I . NaH14C0, was infused at a constant rate either subcutaneously or intraperitoneally into sheep kept in a uniform environment and consuming a standard amount of food each hour.After 3 h of infusion the specific radioactivity (SR) of COe obtained by acidification of samples of blood taken from the right atrium showed no significant change with time until after 22 h when the infusions were stopped.2. Entry rates of COa (I/h) in sheep were estimated from the ratio of rate of infusion of "CO, as NaH14C08 (,uCi/h) to the SR of COB (&ill) in samples of jugular venous blood, urine and exhaled gas taken after infusions had been in progress for at least 3 h. Concurrently, rates of energy expenditure were calculated from the gaseous exchanges over periods of 60-90 min which were determined for grazing sheep by use of re-entrant tracheal cannulation and metering and analysis of exhaled gas, or for housed sheep by measurement in respiration chambers. 3. Significant positive relationships were found between entry rate, the independent variable, and the contemporary energy expenditure. Equations obtained for grazing sheep were not significantly different from those obtained for other, housed, sheep when both groups were infused subcutaneously and COz for assay of radioactivity was obtained in all instances from blood, or in all instances from urine.4. With subcutaneous infusion the equation relating energy expenditure to entry rate estimated from the SR of urinary COa differed significantly from that where entry rate was derived from radioactivity assays of blood or exhaled COz. There was also a significant difference between two equations where entry rates were determined from the SR of blood COz but route of infusion was in the one instance intravenous and in the other was subcutaneous. Reasons for the differences between equations are discussed. 5. Changes in the rates of energy expenditure of sheep effected by intermittent exercise on a treadmill were reflected in changes in COB entry rates. Values for the energy cost of horizontal locomotion by the sheep were derived from the entry rates and were similar in magnitude to those reported by other workers.6. I t is suggested that the method of determining energy expenditure from COz entry rate may be adapted for use on many species of animal in a variety of environments.The usual methods of animal calorimetry require close restraint of the subjects. Portable equipment for determining the energy expenditure of humans from respiratory gaseous exchanges has been adapted for use on domestic livestock, but the use of a face mask prevents feeding and may restrict other activities. An alternative is

Volatile fatty acid isolated from nine samples of peripheral blood from four cows contained, on a molecular basis, from 90.0 to 97.0 per cent. of acetic acid (mean 93.3 per cent.). The remainder comprised, as mean values, propionic acid, 2.39 per cent.; butyric acid, 2.61 per cent.; and a group of at least three acids between butyric and octanoic, 1.84 per cent. The significance of the high proportion of acetic acid in the volatile fatty acid of bovine peripheral blood is discussed. Only traces of esterified acids lower than octanoic could be found in bovine blood lipides. Volatile fatty acids were found also in the blood of the rabbit, guinea pig, horse, and pig and in human plasma. Here again a high proportion of acetic acid was recorded. Volatile fatty acid isolated from nine samples of ruminal contents from two cows contained on a molecular basis from 52.3 to 69.0 per cent. of acetic acid (mean 60.0 per cent.). The remainder comprised, as mean values, propionic acid, 21.8 per cent.; butyric acid, 14.4 per cent.; and acids higher than butyric (apparently largely valeric and hesanoic), 3.8 per cent. This limited number of analyses indicated no gross effect of type of feed on the proportion of the acids in the rumen.

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