utilization for protein synthesis of 2ketoisocaproate relative to utilization of leucine as estimated from exhalation of labelled co2 1 we have previously shown that the ratio rwbp of incorporation of label from 2ketoisocaproate kic into the leucine of wholebody protein to the simultaneous incorporation of label from leucine itself into protein is a measure of the nutritional efficiency of kic as a substitute for leucine 2 in order to determine whether rwbp can be estimated indirectly from measurement of labelled co2 excretion rats were injected orally or intravenously with 453hleucine and either 114cleucine or 114ckic expired co2 was collected for 6 h 3 the results show that 914 of kic underwent firstpass oxidation after oral administration when isotopes were given intravenously the mean rate of excretion of 14co2 from kic after 20 min remained 18 times the mean rate of excretion of 14co2 from leucine 4 mean rwbp measured in wholebody protein in rats given isotopes orally or intravenously along with small or large doses of carriers was the same as mean rwbp estimated from mean cumulative co2 excretion 5 we conclude 1 that nutritional efficiency of kic relative to leucine can be estimated from measurement of labelled co2 excretion and 2 that the relative inefficiency of kic as a substitute for leucine in the rat is attributable to firstpass oxidation of 914 when given orally and 80 greater susceptibility to systemic oxidation than leucine