Duckweed colonies were grown on 1 1 of nutrient solution supplied with 10 gM L-[14C]leucine or with 25 I-tM L-[14C]valine. Under these conditions the exogenously supplied amino acid did not inhibit growth, but caused in the plants a moderately increased pool of that amino acid, which remained essentially constant during the culture period. The effect of the increased pool of valine or leucine on the biosynthesis of these amino acids was determined from isotope dilution in the protein-bound valine and-or leucine. An increase in the leucine pool from 1.1 to 5.0 nmol mg-1 dry weight resulted in a 21% reduction of metabolite flow through the common part of the valine-leucine biosynthetic pathway; leucine synthesis was reduced by 35%, but valine synthesis by only 5% and isoleucine synthesis was apparently unaffected. An increase in the valine pool from 3.2 to 6.6 nmol mg-1 dry weight reduced the metabolite flow through the valine-leucine pathway by 48%, valine synthesis by 70%, and leucine synthesis from pyruvate by 29%, which was compensated by leucine synthesis from exogenous valine, whereas the synthesis of isoleucine was not changed. It is concluded that the biosynthesis of valine and leucine is mainly controlled by feedback inhibition of acetohydroxyacid synthetase. In vivo, the feedback inhibition can be exerted in such a way that synthesis of acetolactate (the precursor of valine and leucine) is appreciably reduced, whereas synthesis of acetohydroxybutyrate (the isoleucine precursor) is not inhibited.