The synthesis of several derivatives of a xenobiotic (phenoxyalkanecarboxylic acid) and an alpha-amino acid (L-lysine) is described. Various substituents were introduced in the aromatic ring of the phenoxyalkanecarboxylic acid and the side-chain was modified. They were tested for their effect on the transport of a neutral (L-threonine), an acidic (L-glutamic acid) and a basic (L-lysine) amino acid, and a sugar (sucrose). All compounds markedly inhibited threonine uptake by leaf tissues of Vicia faba L.--and more specifically phloem loading--with two exceptions, when the aromatic ring was substituted in the 4-position by a primary amino group or when D-lysine was used instead of L-lysine. By contrast, the addition of a chlorine atom in the 4-position of the aromatic ring enhanced the inhibitory activity. Similar results were obtained for inhibition of glutamate uptake and, to a lesser extent, for lysine uptake. pH dependence of the inhibitory activity as well as electrophysiological data indicate that permeases mediating active transport of amino acids are the target of these conjugates. These, in addition to other data obtained with other xenobiotics, show that the amino acid carrier system is capable of recognizing a wide range of conjugates of various sizes, structures and octanol/water partition coefficients.