Photosynthesis, respiration, and other processes produce reactive oxygen species (ROS) that can cause oxidative modifications to proteins, lipids, and DNA. The production of ROS increases under stress conditions, causing oxidative damage and impairment of normal metabolism. In this work, oxidative damage to various subcellular compartments (i.e. chloroplasts, mitochondria, and peroxisomes) was studied in two cultivars of wheat differing in ascorbic acid content, and growing under good irrigation or drought. In well-watered plants, mitochondria contained 9-28-fold higher concentrations of oxidatively modified proteins than chloroplasts or peroxisomes. In general, oxidative damage to proteins was more intense in the cultivar with the lower content of ascorbic acid, particularly in the chloroplast stroma. Water stress caused a marked increase in oxidative damage to proteins, particularly in mitochondria and peroxisomes. These results indicate that mitochondria are the main target for oxidative damage to proteins under well-irrigated and drought conditions.Instituto de Fisiología VegetalFacultad de Ciencias Naturales y MuseoFacultad de Ciencias Agrarias y Forestales