Abstract Up to 22% of the [ 3H]leucine-labeled proteins synthesized chloroplasts in the light was degraded during subsequent incubation for 20–40 min. The degradation of these radioactive proteins was more rapid in the light than in the dark and was at least 2-fold greater in the presence of 5 mM ATP in light or darkness. Exogenous amino acids did not influence degradation rates, although they promoted protein synthesis. Overall, proteins from thylakoid and stromal fractions were degraded at comparable rates. Analysis by electrophoresis in denaturing polyacrylamide gels revealed that many proteins decreased in both fractions. Certain low molecular mass stromal proteins were lost almost completely during a 90 min incubation in the presence of ATP, while others were unaffected or decreased only slightly. Thus Chloroplasts, like eukaryotic and prokaryotic cells and mithochondria, contain an ATP-stimulated proteolytic system.