The etioplast-chloroplast transformation has been followed in sunflower cotyledons. After few days of dark growth these contained etioplasts, whose internal membranes appeared to be of the LS (Lightly Stained) type. The ultrastructural changes occurred in the usual way but the involved membranes remained of the LS type until the formation of wellorganized chloroplasts. Only at a subsequent stage the thylakoid membranes changed their staining showing a strong contrast. As regards the chlorophyll synthesis, after an early lag phase it increased at the same time as the formation of thylakoids. The above confirms the chloroplast ontogenetic model previously proposed for sunflower grown under natural conditions and strengthens the assumption of ours that the insertion of chlorophyllous pigments can already take place in the LS-membranes.