Research carried out into successively older sunflower leaves has allowed us to follow the different ontogenetic stages leading to the building up of a well-differentiated chloroplast. In all the intrathylakoid compartments of the young plastids there is very electron-dense material whose accumulation causes the dilatation of some thylakoid profiles and, consequently, the forming of roundish bodies. The thylakoid membranes appear as “Lithtly Stained Membranes” which, according to the most recent interpretation, are considered as incompletely organized ones. At a subsequent stage there is an increase of the thylakoid membrane staining along with a decrease of the electron-density of the intrathylakoid compartments that, at the end of the ontogenetic process, appear electrontransparent and flattened. Therefore, the intrathylakoid material appears to have been used up in the forming of well-organized membranes. This particular situation, present in the early ontogenetic stages of the plastids, may be attributed to an initial disequilibrium between the synthesis speed of membranal material and the capability to build up well-organized membranes.