Abstract.Aging of chloroplasts bothin vivoandin vitrocauses a considerable loss in the 2,6‐dichlorophenol indophenol (DCPIP)‐Hill reaction with water as electron donor. The loss can be reduced by exogenous electron donors like diphenyl carbazide (DPC). suggestive of aging‐induced damage of the oxygen evolving system. Aging also brings about a considerable loss in methylviologen (MV) reduction mediated by Photosystem I (PS I) of chloroplasts with an ascorbate‐DCPIP couple as the electron donating system.The loss in the electron transport ability of the plastids is faster duringin vitrocompared toin vivoaging of the chloroplasts.Light protects the photo‐electron transport ability of chloroplasts during aging of intact leaves in contrast to its action during aging of the isolated o