Commercially available dry dog foods are often manufactured using extrusion cooking. Normal extrusion conditions can promote Maillard reactions involving the amino acid lysine. This non-enzymatic browning reaction occurs between a carbonyl group and an amino group in a diet. This study investigated the influence of a number of extrusion parameters for the manufacture of a dry dog food on the total and reactive lysine content. A 3×2×2 factorial design was used, with mass temperature (110°C, 130°C, and 150°C), diet moisture content (200 and 300 g/kg), and extrusion runs (once or twice) as variables. A diet formulated with wheat, maize, rice, chicken meat, poultry meal, and barley as the main ingredients (g/100 g dry matter |DM], including 23.3 crude protein, 11.3 crude fat, 53.1 starch, 0.6 ash, and 0.2 crude fiber) was extruded using a co-rotating twin-screw extruder (APV Baker MPF 50 [APV Baker, Peterborough, England]; length:diameter ratio: 25; screw speed: 15%; torque: 5% to 6%). Total lysine was measured by high-performance liquid chromatography (HPLC), and reactive lysine was determined by conversion of lysine by O-methylisourea (OMIU) to homoarginine, as described by Moughan and Rutherfurd.1 Total lysine content in the unprocessed diet was 0.98, and the OMIU-reactive lysine content was 0.70 of the total lysine content (0.69 g/100 g DM). The first extrusion run increased the ratio of OMIU-reactive lysine:total lysine, depending on extrusion temperature and moisture content. The ratio increased because of increased extrusion temperature. The higher moisture content lessened the increase in the ratio caused by increased temperature. Total lysine and other amino acids were unaffected by extrusion. The second extrusion run resulted in a lowering of the ratio of OMIU-reactive lysine:total lysine ratio when the ratio was close to 1.0. Ratios below 1.0 were further increased as a result of the second extrusion. Unprocessed canine diets can already have damaged lysine because of the use of heat-treated ingredients. Extrusion can have a variable effect on the OMIU-reactive lysine content of canine diets.