The pattern of nitrogen assimilation in soybean plants treated with a herbicide that inhibits branched-chain amino acid biosynthesis was evaluated by (15)N isotopic analysis. The herbicide imazethapyr caused a strong decrease in nitrate uptake by roots, partly due to a reduced stomatal conductance. The inhibition of (15)N uptake was accompanied by a decrease in the (15)N content in the plant and, concomitantly, an inhibition of translocation to the shoot. Imazethapyr inhibited nitrate reductase activity in leaves and roots. Among all parameters studied, "de novo" synthesis of proteins was the first parameter of the N assimilation metabolism affected by the herbicide. These results show that this class of herbicides totally damages N metabolism and indicates a regulatory effect on N uptake and translocation that would be mediated by the increase in free amino acid pool provoked by the inhibition of branched-chain amino acid biosynthesis.