To examine the influence of lysine imbalance on urinary excretion of lysine metabolites and liver xanthine oxidase activity in young rats, lysine imbalance in rats was observed by 8% casein diet supplemented with 1 2, 3 or 4% L-lysine hydrochloride. During the feeding for 14 days, whole urine of each rat was collected daily, and at theend of the feeding, the blood serum and liver were taken for the analysis. The growth of rats and efficiency of feed utilization were significantly increased by the addition of 10% lysine, while the more addition of it gradually decreased the growth rate and liver xanthine oxidase activity. The amount of urinary excretion of lysine, α-aminoadipic acid and glutamic acid were gradually increased in proportion to the supplemented lysine, and it was suggestedthat the urinary α-aminoadipic acid, an unusual amino acid in normal urine, was a criterion to the grade of development of lysine imbalance.