L-alpha-Aminooxy-beta-phenylpropionic acid (AOPP), a potent competitive inhibitor of phenylalanine ammonia-lyase (PAL), blocked light-induced phenylpropanoid synthesis in excised buckwheat hypocotyls and produced an up to 40-fold increase in the endogenous phenylalanine concentration, while the level of all other amino acids was hardly affected. After a 24 h incubation in the light in the presence of 0.3 or 1 mM AOPP phenylalanine alone constituted about 25% of the total soluble amino acids, compared to appr. 1% in the controls. In the presence of AOPP illuminated hypocotyls accumulated nearly 3 times more phenylalanine than hypocotyls kept in the dark, indicating an enhancing effect of light on the flow of carbon through the shikimate pathway. Exogenously added [14C]phenylalanine was extensively metabolized by control tissue, but accumulated in AOPP treated tissue. In the presence of AOPP radioactivity from [14C]shikimate accumulated predominantly in phenylalanine, and the flow of shikimate into tyrosine and phenylalanine was not affected by the inhibitor. Therefore, under these conditions no feedback control of phenylalanine and tyrosine synthesis from shikimate is apparent in buckwheat hypocotyls.