The influx of L-lysine into apical vesicles from the chicken jejunum occurs through two systems, one with low Michaelis constant (K_m) and features of system b^0,+ and the other with relatively high K_m for L-lysine and with properties of system y⁺. In the present study the effect of a lysine-enriched diet (Lys, containing 68 g L-lysine/kg dietary protein, control animals 48 g/kg) on L-lysine uptake through both transport systems was investigated. Results show that 1) lysine enrichment had no effect on either body weight or the efficiency of food utilization. 2) In Lys-fed animals, the mediated L-lysine influx was best fitted to the two-system model with y⁺ and b^0,+ activity. 3) In the presence of an Na⁺ gradient, total L-lysine uptake is significantly higher in Lys-fed animals than in control birds (about 40% increase). 4) Lys diet increases K_mb^0,+ 6-fold (KSCN gradient) and 12-fold (NaSCN gradient) and maximum velocity (V_max) by 6- and 20-fold, respectively. The effects of Lys enrichment on the y⁺-like system are only observed on the V_max and in the presence of a Na⁺ gradient (30% increase). 5) Na⁺ is involved in the activation of the transport process in the Lys-fed chickens, but there is no correlation between external Na⁺ concentration and L-lysine influx. In conclusion, both b^0,+- and y⁺-like transport systems are upregulated by dietary lysine but with different kinetic profiles; the high-capacity y⁺-like carrier shows a V_max increase without changes in K_m, whereas the low-capacity b^0,+-like system shows an increase in V_max as well as in the K_m.