Summary To get insight in the metabolism of the latex-specific amino acid m-tyrosine of Euphorbia myrsinites L., m-tyrosine-[2-14C] was fed to shoots of this plant. The compound damages the plants and is selectively excreted into the latex. In the tissue a partial oxidation occurs to polyphenols and melaninlike products. Another part of the m-tyrosine is transformed in the tissue and the latex to 1-methyl-6-hydroxy-1,2,3,4-tetrahydroisochinoline-3-carboxylic acid by Mannich-condensation with acetaldehyde or its biochemical equivalent. The same reaction occurs spontaneously in the isolated latex. m-Tyrosine-[2-14C] and phenylalanine-[2-14C] fed to sprouts of Euphorbia helioscopia L. are converted to m-hydroxyphenylglycine, occurring in the latex of this species. The unphysiological amino acid is taken up by the plant only in a small degree compared with the proteinogenic amino acid phenylalanine and is preferentially eliminated into the latex. Phenylalanine on its way into the latex is markedly degraded. Incubation studies with isolated latex from Euphorbia helioscopia L. and Euphorbia lathyris L. showed that m-tyrosine is not transformed and not incorporated into m-hydroxyphenylglycine and 3,4-dihydroxyphenylalanine respectively, amino acids occurring in the latex of these species. Shikimic acid[U-14C] is incorporated in a very small amount into m-tyrosine in the isolated latex of Euphorbia myrsinites L., especially after the increase of the pH value about the neutral point. These results are discussed in relation to the physiological significance of the amino acids from latex and of the latex itself and its particular properties.