Preparations of isolated chloroplasts show a reduction of photophosphorylation activity due to ageing even when stored in the dark at 0-4°C in a stabilizing medium. The rate of activity loss depends strongly upon the type of photophosphorylation. With non-cyclic photophosphorylation (ferricyanide or NADP-ferredoxin) reduction of activity is rapid immediately after isolation of chloroplasts, whereas it decreases to much slower rates after 3-4 hours. In the presence of PMS, which under our reaction conditions serves as electron acceptor for PS-2 and electron donor for PS-1, photophosphorylation activity is hardly affected by the increasing age of chloroplasts. However, upon addition of low concentrations of DCMU, reduction of this phosphorylation activity due to ageing shows the same kinetics as non-cyclic photophosphorylation. Photophosphorylation of PS-1 (reduced DCPIP-MVDCMU) is hardly affected by the ageing of chloroplasts. Abbreviations: ATP: adcnosinc-5’-triphosphate, BSA: bovine serum albumin, DCMU: 3(3-4)dichlorophenyl-l, 1-dimethyl urea, DCPIP: 2,6-dichlorophenolindophenol, MV: methylviologen, NADP; nicotinamide adenine dinucleotide phosphate, 32P: the radioactive isotope phosphorus-32, PS-1 (2): photosystem 1 (2), PMS: phenazine methosulphate.