In the chicken intestine, L-methionine is transported by systems that are specific for neutral amino acids (L- and B-like) and by systems that can also transport cationic amino acids (y⁺m and b^0,+-like). These four uptake pathways have been investigated in brush-border membrane vesicles from the jejunum of chickens fed a diet enriched with 0.4% L-methionine. Methionine supplementation from the 1st to the 6th wk of age has no effect on body weight or on the efficiency of food utilization. The kinetic analysis of L-methionine influx across the transport systems specific for neutral amino acids shows, for system L, no dietary effect on the Michaelis constant (K_m) and a 30% reduction in maximal velocity (V_max); for system B it shows a decrease in K_m (30%) and in V_max (51%). Transport systems shared by cationic and neutral amino acids show no dietary effect on b^0,+ activity and a significant reduction in y⁺m V_max, similar for L-methionine and L-lysine, both in the absence and in the presence of Na⁺ (L-methionine, 30 and 26% reduction; L-lysine, 19 and 28% reduction, respectively). The downregulation induced by L-methionine supplementation may be an adaptive response to reduce the risk of intoxication by dietary excess of L-methionine. These results support the view that the toxicity of the supplemented substrate can be an important factor in the regulation of amino acid transport by dietary content.