These studies examined the lysine requirement of the immature Beagle dog fed a purified l-amino acid diet. The immature female dog was found to require between 0.461 and 0.577% dietary lysine for optimum growth and nitrogen balance. Plasma and urinary urea were significantly increased in female dogs fed 0.461% or less dietary lysine. Maximum growth and nitrogen balance occurred in immature male dogs fed 0.577% or more lysine. Consumption of diets containing excess dietary lysine (1.73%) significantly reduced growth in both immature male and female dogs. Varying dietary lipid from 5 to 20% did not appear to alter the lysine requirement of the immature male dog. Increasing the nitrogen content of the diet from approximately 14% crude protein to 28% accentuated the symptoms of low lysine intakes. The lysine requirement of the immature dog appears to be lower than that reported for the growing pig or rat.lysine dogs urea nitrogen amino acids