In developing primary leaves of illuminated oat seedlings, the accumulation of C-glycosylflavones — derivatives of vitexin and isovitexin — is closely related to activity changes of the enzymes phenylalanine-, tyrosine ammonia-lyase (PAL, TAL) and chalcone-flavanone isomerase (CFI). The isomerase activity was determined in vitro with the substrate 2′,4,4′,6′-tetrahydroxy-chalcone (CFITetra see schema 1), which is converted to the corresponding 5,7,4′-trihydroxy-flavanone naringenin, as the only reaction product to be detected. Enzymatic activity measured with 2′,4,4′-trihydroxychalcone as substrate (CFITri) did not show a direct relation with flavone accumulation which was observed using tetrahydroxychalcone as substrate. On the 4th day of germination, when greening of the primary leaf begins (stage 1–2, fig. 13), a drastic increase of all 3 enzymatic activities per organ is observed, whereas specific activities show the highest values in this early phase of development. At the same time flavone synthesis occurs in an eruptive manner: the maximal rate is found on the 5th day of growth when PAL and TAL reach maxima (stage 2–3). Afterwards, during the decrease of ammonia-lyase activity, flavone content is further increased — concomitant with the CFITetra-activity — for about 24 hours when a stationary phase is reached. In this stage (stage 4) the rise of isomerase activity also declines. These observations lead to the assumption that CFITetra plays an important role in the specific flavone metabolism in primary leaves of Avena, and that naringenin, the 5-hydroxylated flavanone, is probably the in vivo precursor of the C-glycosylflavones with 5-OH substitution. In comparison with the light-grown primary leaves, the growth of etiolated leaves is retarded as is the rate of flavone synthesis. The increase in the activities of PAL, TAL and CFI also shows a similar delay, but the ammonia-lyases reach — in contrast to the isomerase — identical maximal activities as found with green leaves which are 1 day younger. In addition, neither a light-stimulation of extractable Pal nor of Tal is observed during illumination for 1 day of etiolated seedlings of different age (fig. 7). On the other hand, isomerase activity is stimulated in these experiments parallel to the flavone accumulation (fig. 14). These results support the suggestion that PAL- or TAL-reactions are not rate limiting in the biosynthesis of Avena-glycosylflavones, but that enzymes catalyzing subsequent reactions, mainly C15-enzymes including CFI, control the rate of flavone synthesis. The data suggest — as in other plants — a different regulation of «C9«- and «C15«-enzymes which are responsible for the common phenylpropanoid metabolism or the specific glycosylflavone synthesis, respectively, in primary leaves of Avena sativa. In contrast to the primary leaves, no comparable connections can be find with cole-optiles and mesocotyls. Enzyme activities as measured in vitro under vmax-conditions are discussed in relation to the accumulating amount of flavonoids and their possible «turnover«. The results presented here are a suitable base for further studies on the regulation of flavonoid metabolism in Avena, in which emphasis will be laid on the involvement of subcellular compartments such as plastids (see Weissenböck and Effertz, 1974).