To analyze the potential of the active oxygen-scavenging system of chloroplasts, we introduced Escherichia coli catalase into tobacco chloroplasts, Photosynthesis of transgenic plants was tolerant to high irradiance under drought conditions, while the wild plants suffered severe damage in photosynthesis under the same conditions. Irrespective of responses to the stress, ascorbate peroxidase was completely inactivated both in the transgenic and wild-type plants. These findings are contrary to the established idea that the ascorbate peroxidase-mediated antioxidative system protects chloroplasts from oxidative stress. (C) 1998 Federation of European Biochemical Societies.