Abstract Eight healthy individuals were fasted for 72 hours. The concentrations of the branched chain keto acids (BCKA), branched chain amino acids (BCAA), C peptide, and glucagon were determined in peripheral venous blood. α-ketoisocaproic acid, α-keto-β-methyl-n-valeric acid, and α-ketoisovaleric acid increased significantly within 36 hours along with the corresponding amino acids. After 60 hours of starvation, the concentrations of BCKA and BCAA declined despite the fact that the subjects were still in the fasting state. These changes were accompanied by a decrease in the concentrations of C peptide and an increase in glucagon levels. It is suggested that in starving man (1) insulinopenia may contribute to the rise in BCKA concentrations and that (2) the increase in BCKA may be a mechanism to reduce proteolysis.