AbstractThe incorporation of orally administered 1‐14C‐α‐ketoisocaproate into the leucine of proteins in rats was compared with the incorporation of [3H]leucine itself administered simultaneously and expressed as a ratio, R. This ratio in whole body protein has been shown to be approximately equal to the nutritional efficiency of α‐ketoisocaproate as a dietary substitute for leucine. In normal rats on a 14% protein diet, R in whole body protein (0.30 ± 0.01) and in the protein of various organs was the same whether the isotopes were given by single injection or 6‐hr constant infusion. Thus, both techniques yield the same time‐independent parameter, R, which measures the relative efficiency of α‐ketoisocaproate as a substitute for leucine. R varied between organs as follows: liver (0.22 ± 0.01)