The synthesis of plastidquinones was studied from leaf development until autumnal leaf degeneration in sun and shade leaves of Fagus silvatica and compared with chlorophyll and carotinoid synthesis. 1. The lipoquinone content of Fagus chloroplasts increases steadily with increasing age of leaf tissue. This lipoquinone accumulation takes place in sun and shade leaves, it is however much more expressed in sun leaves. On a chlorophyll or a leaf square (100 cm 2 ) basis the latter possess a several times (3 to 9 x) higher lipoquinone content than shade leaves. 2. The augmentation of the plastidquinone level in both leaf types is mainly due to the synthesis of the reduced benzoquinone forms, plastohydroquinone 45 and the chromanol a-tocopherol. The larger part of the reduced lipoquinones represent excess amounts which are deposited extrathylakoidal in the osmiophilic plastoglobuli of the plastid stroma. 3. The oxidized benzoquinones, plastoquinone 45 and α-tocoquinone, as well as the naphthoquinone vitamin K 1 (phylloquinone) are present in much lower concentration. Their synthesis parallels that of chlorophylls and carotinoids. They are formed either not in excess (vitamin K 1 ) or in very low excess amounts (α-tocoquinone, plastoquinone) . These oxidized quinone forms are in turn preferably located together with chlorophylls and carotinoids in the photochemically active thylakoids. 4. The production of excess plastidquinones (mainly reduced benzoquinones) starts very early during leaf development. The main synthesis phase begins after the end of chlorophyll and thylakoid synthesis and continues in senescent Fagus leaves during the breakdown of chlorophyll and thylakoids until the early yellow stage of sun and shade leaves. In senescent leaves the level of the oxidized benzoquinones, plastoquinone and α-tocoquinone, increases slowly by oxidation of plastohydroquinone and α-tocopherol, respectively. 5. The differences between sun and shade leaves in the total and relative amounts of the various lipid classes such as chlorophylls, carotinoids, benzoquinones and the naphthoquinone K 1 (100 cm 2 leaf square, dry weight or chlorophyll a as reference system) are not yet significant in May, several days after leaf unfolding, but get established thereafter. 6. By August the benzoquinone content of sun leaves reaches even that of chlorophyll a, whereas in shade leaves the lipophilic benzoquinones make up only 20 to 30% of the chlorophyll a content. Shade leaves exhibit a benzoquinone level which is little lower than that of carotinoids. Sun leaves, in contrast, contain a benzoquinone content which is by ca. 4 times higher than the carotinoid content. The ratio benzoquinones to vitamin K 1 amounts 60 - 70 in shade leaves and 100 - 120; n sun leaves. In addition the two lipophilic redox systems plastoquinone/plastohydroquinone 45 and α-tocoquinone/α-tocopherol are present in sun leaves to a higher extent in the reduced state (84% and 96%) than in shade leaves (60% and 90% respectively). 7. From the results it is assumed, that the steady increase of the benzoquinone level with increasing age of plant tissue may be correlated with thylakoid turn-over. The possibility, that the enormous benzoquinone accumulation in sun leaves is mainly caused by higher light intensities in connection with a reduced protein synthesis is discussed.