During growth of the primary leaves of Arena sativa L., the distribution of extractable Lphenylalanine ammonia-lyase (PAL, EC 4.3.1.5) and chalcone-flavanone isomerase (CFI, EC 5.5.1.6) activities in distinct leaf sections (top section, medium section and meristematic basal section) and in the epidermal and mesophyll tissues were investigated in relation to C-glycosylflavone accumulation. Characteristic changes have been observed in the levels of PAL and CFI activities within the three leaf sections, depending upon their stage of development. An increase in both enzyme activities accompanies a strong flavone accumulation in the section of the leaf that derives from the basal meristem. Highest specific PAL activity is localized in the meristem itself, which is poor in both flavones and CFI activity. Total flavone accumulation was found to be nearly the same in all three leaf tissues, lower and upper epidermis and mesophyll. Similarly, PAL activity is distributed about equally in these tissues in young leaves; in older ones, activity is relatively higher in the lower leaf epidermis. In contrast, CFI is found to be localized almost entirely in the mesophyll and not in the epiderms. Therefore the question arises whether CFI is involved at all in flavone metabolism and whether it may represent, as a marker enzyme, the localization of other specific C15-enzymes of the flavonoid biosynthetic pathway in oat primary leaves.