Phenylalanine-G-14C and tyrosine-G-14C were administered to shoots taken from the normal and variant forms of Eucalyptus sideroxylon and variant forms of E. dalrympleana and E. guilfoylet. The conversion of these amino acids to polyphenols was much less with the normal form than with the variant form. The degree of incorporation of radioactivity into polyphenols was in the order of chlorogenic and p-coumarylquinic acids, catechin, dihydrokaempferol rhamnoside, flavonol glycosides and stilbenes. The biosynthesis of different classes of monomeric polyphenols appears to proceed by parallel pathways and the activity of a compound is dependent not only on its relative position in a biosynthetic pathway but on its relative rate of conversion to polymers in the dynamic plant system. Evidence indicates that quercetin is formed by hydroxylation of kaempferol, that rhamnosides are formed more readily than glucosides and that in the formation of disaccharides, the monosaccharides are added one at a time. It is postulated that cyclization of acetate units in flavonoids and stilbenes is a rate-determining step which results in an accumulation of p-coumarylquinic and chlorogenic acids when an unusually large amount of material passes through the prephenic acid pathway. Tyrosine was largely converted to sugars and apparently acetate units were degradation products; no evidence for tyrase was found in E. sideroxylon leaves. The conversion of phenylalanine to sugar and acetate also took place but to a much smaller extent.