The effect of fifteen amino acids, supplied singly, on the growth of isolated germinating barley embryos in the presence of nitrate has been studied. The L forms of lysine, arginine, tyrosine, proline, threonine, methionine, leucine, and valine at concentrations of either 1 or 2 mM have been found to inhibit fresh-weight accumulation. The inhibition by valine is relieved by further addition of isoleucine and that of leucine by the addition of both isoleucine and valine. These interrelations have been interpreted as suggesting that leucine and valine can inhibit acetolactate and acetohydrorybutyrate synthesis. The inhibition of tyrosine can be relieved by phenylalanine and that of lysine by ornithine or arginine. The possible reasons for these interrelationships are discussed.