1. NaH14CO3, 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 14CO2, as NaH14C08 (µCi/h) to the SR of CO2 (µCi/l) 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 meter-ing 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 CO2. There was also a significant differ-ence between two equations where entry rates were determined from the SR of blood CO2 but route of infusion was in the one instance intravenous and in the other was sub-cutaneous. 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 CO2 entry rates. Values for the energy cost of hori-zontal locomotion by the sheep were derived from the entry rates and were similar in magni-tude 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.