The alteration in both the lipid composition and chlorophyll proteins obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of detergent solubilized thylakoids was investigated on differentially aged spinach (Spinacia oleracea L.) chloroplasts. Freshly isolated material demonstrated seven major bands upon electrophoretic fractionation. Membranes aged in vitro showed a diminution and/or a disappearance of some bands concomitant with changes in the acyl lipid composition of these membranes. The extent of these changes was influenced by the purity of the preparation. Low temperature fluorescence measurements (77K) showed that upon aging, the photochemical capacity of photosystem II decreased prior to alterations in the molecular organization of the photochemical apparatus as indicated by the energy distribution between the two photosystems.The results are discussed in terms of whether the series of changes in the acyl lipid composition upon aging is related to the variations in the electrophoretic pattern of the chlorophyll proteins.