Young developing seedlings of Mimulus cardinalis Douglas (Scrophulariaceae) were grown in Hoagland's medium supplemented with lysine (LYS; 1.0 mM), threonine (THR; 1.0 mM),LYSq-THR (1.0 mMea.) or Lu (MET; 1.0 1.0-0.1raM, respectively). LYS or THR reduced the total amount of protein in the developing seedlings and when added in combination completely inhibited the accumulation of protein after 7 days. The combination of LYSq-THI~q-MET enhanced the accumulation of protein. When MET was added to seedlings grown on LYSq-THI~ for 9 days, the rate of protein accumulation reverted to normal. Studies using [14C]leucine demonstrated similar inhibitory patterns on protein synthesis. The effects of LYS and THR on the distribution of [14C]aspar~ate into the aspartate amino acids ineluding MET were studied. Lu and THI~, singly and synergistically when in combination, inhibited the synthesis of MET. Thus the simultaneous application of LYS and THR can be interpreted as producing an artificially induced MET auxotroph. These results indicate that LYS and THR act singly and together in vivo to inhibit the accumulation of soluble LYS, MET and THR in a manner indicative of endproduct inhibition. Indirect evidence for end-product control of alanine, arginine, glycine, phenylalanine, proline, tryptephan, tyrosine and valine is also given.