The present paper offers a dual 14CO2 breath test approach to study the metabolic utilization of free amino acids in the body. Using the carboxyl-[14C]isotopomer of an amino acid as the test substrate the percentage recovery of the isotope as 14CO2 reflects which part of the labelled amino acid flux has been decarboxylated. The residual C fragments may flow to total oxidation at least to the level recovered for the universal [14C]isotopomer. In the case that recovery for total oxidation is less than for decarboxylation, part of the [14C]fragments are retained in the body by either exchange or non-oxidative pathways. Utilization of tyrosine and leucine was measured in the post-absorptive phase in adult rats, conditioned on isoenergetic diets containing 210, 75 or 0 g protein/kg. It was shown that the level of dietary protein exerts an influence on both decarboxylation and total oxidation. Although the responses of leucine and tyrosine were not different for total oxidation, there was a difference between the amino acids in their relative rate of decarboxylation. That this dual 14CO2 breath test approach can be used as a tool to evaluate whether the protein and amino acid supply has been adequate to support actual requirements is discussed.