Lysine is frequently a first- or second-limiting amino acid in poultry diets. Improving the efficiency of lysine use for protein synthesis would effectively lower the lysine requirement and decrease feed costs. Understanding how lysine is degraded and how the degradation is regulated would identify potential molecular targets for interventions to decrease lysine degradation. To better understand lysine degradation in poultry, 3 experiments were conducted. In experiment 1, one-day-old chicks were fed 1.07, 1.25, 1.73, or 3.28% dietary lysine for 2 wk. In experiments 2 and 3, fourteen-day-old chicks were fed 1.07 or 1.25% dietary lysine for 2 wk. Measures of liver lysine catabolism including lysine α-ketoglutarate reductase (LKR) and lysine oxidation (LOX) were assessed. The α-aminoadipate δ-semialdehyde synthase (AASS) is a bifunctional enzyme composed of both LKR and saccharopine dehydrogenase activities, and the relative abundance of this protein and mRNA were likewise assessed. Moreover, potential alternative pathways of lysine catabolism that depend on l-amino acid oxidase (AAOX) and on lysyl oxidase (LYLOX) were considered. In experiment 1, chicks fed lysine-deficient diets had decreased (P 0.05) LOX as measured in vitro. In experiments 2 and 3, chicks 28 d of age did not decrease LKR activity (P > 0.05) in response to a lysine-deficient diet. No changes in AASS protein abundance or mRNA were detected. Likewise, no differences in the mRNA abundances of AAOX or LYLOX were detected. The activity of AAOX did increase (P < 0.05) in birds fed a lysine-adequate diets compared with those fed a lysine-deficient diet. Based on kinetic parameters and assumed concentrations, AAOX could account for about 20% of liver lysine oxidation in avians.