As part of an effort to explain the leucine-induced depressions of plasma isoleucine and valine concentrations, and the concomitant stimulation of valine oxidation in vivo, branched-chain alpha-keto acid dehydrogenase (BCKAD) activity was measured in livers from rats that were fed for only 6 h/d large quantities of individual amino acids in a low protein diet. Preincubation of homogenates with buffer containing Mg2+ and Ca2+ allowed estimation of fully active complex. Cytosolic and mitochondrial branched-chain-amino-acid aminotransferase (BCAAT) activities were also measured in livers of rats fed an excess of leucine. The percentage of BCKAD in the active form in livers of rats fed the low protein diet containing an excess of leucine, isoleucine, valine or phenylalanine for 2 d was double that of rats fed the low protein control diet (control, leucine, isoleucine, valine and phenylalanine groups having, respectively, 45 +/- 2, 85 +/- 7, 85 +/- 3, 95 +/- 5, and 81 +/- 4% of hepatic BCKAD in the active form). Consumption of a low protein diet containing an excess of leucine had no significant effect on either cytosolic or mitochondrial BCAAT activities of liver. The response of BCKAD in liver can contribute to the leucine-induced stimulation of valine oxidation in vivo but analysis of the results of this study leads to the conclusion that other mechanisms, probably in nonhepatic tissues, must also be involved.