The concentration dependence of the influx of L-lysine in excised roots of Arabidopsis thaliana seedlings was analyzed for the wild-type (WT) and two mutants, rlt11 and raec1, which had been selected as resistant to lysine plus threonine, and to S-2-aminoethyl-L-cysteine, respectively. In the WT three components were resolved: (i) a high-affinity, low-capacity component [K-m=2.2 mu M; V-max=23 nmol .(g FW)(-1). h(-1)]; (ii) a low-affinity, high-capacity component [K-m=159 mu M; V-max=742 nmol .(g FW)(-1). h(-1)]; (iii) a component which is proportional to the external concentration, with a constant of proportionality k=104 nmol .(g FW)(-1). h(-1). mM(-1). The influx of L-lysine in the mutants was lower than in the WT, notably in the concentration range 0.1-0.4 mM, where it was only 7% of that in the WT. In both mutants the reduced influx could be fully attributed to the absence of the low-affinity (high-K-m) component. This component most likely represents the activity of a specific basic-amino-acid transporter, since it was inhibited by several other basic amino acids (arginine, ornithine, hydroxylysine, aminoethylcysteine) but not by L-valine. The high-affinity uptake of L-lysine may be due to the activity of at least two general amino acid transporters, as it was inhibitable by L-valine, and could be further dissected into two components with a high affinity (K-i=1-5 mu M) and a low affinity (K-i=0.5-1 mM) for L-valine, respectively. The rlt11 and raec1 mutant have the same phenotype and the corresponding loci were mapped on chromosome 1, but it is not yet clear whether they are allelic.