Isolated intact spinach chloroplasts were supplied with 14C-labeled CO2, phosphoglycerate, phos-phoenolpyruvate, acetate and mevalonate and the incorporation of radioactivity into β-carotene and plastoquinone-9 assayed. All applied substrates were capable of crossing the chloroplast envelope. Except phosphoenolpyruvate all radioactive precursors were incorporated into β-carotene and also into plastoquinone-9. It is concluded that spinach chloroplasts are autonom in acetyl-coenzyme-A-formation and terpenoid biosynthesis. There is a direct carbon flow from intermediates of the Calvin-cycle via acetyl-coenzyme-A and mevalonic acid existing in the chlo roplast leading to the carotenoids and isoprenoid side-chains of chlorophylls and quinones.