Lysine (Lys) requirement estimated by laying performance and that by response of plasma Lys concentration to changes of dietary Lys in a short period were compared. Laying hens at 39 weeks of age were divided into five groups, housed individually and given free access to the experimental diets and water for 21 days. Egg weight was recorded every day and body weight and feed intake were recorded before every blood sampling time. Blood samples were taken on 1, 2, 4 and 21 days after changing diets for the determination of plasma amino acid concentration. The Lys requirements estimated by egg production rate and egg mass were 0.63 and 0.64%, respectively. The Lys requirements determined by plasma Lys concentration were 0.70, 0.63, 0.62 and 0.64% on 1, 2, 4 and 21 days after changing diets, respectively. All values except for that on the first day after changing diet agreed well with each other and those from laying performance. Though it took about seven days until the low dietary Lys affected the egg production rate, the plasma Lys concentration responded to the changes of dietary Lys at two days. The responded plasma Lys concentration was maintained for 21 days. These results suggested that the Lys requirement could be predicted from plasma Lys concentration on the second day after changing diets.