Exposure to adverse environmental conditions causes oxidative stress in many organisms, leading either to disease and debilitation or to response and tolerance. Mitochondria are a key site of oxidative stress and of cellular response and play important roles in cell survival. We analyzed the response of mitochondria in pea (Pisum sativum) plants to the common stresses associated with drought, cold, and herbicides. These treatments all altered photosynthetic and respiratory rates of pea leaves to various extents, but only herbicides significantly increased lipid peroxidation product accumulation. Mitochondria isolated from the stressed pea plants maintained their electron transport chain activity, but changes were evident in the abundance of uncoupling proteins, non-phosphorylating respiratory pathways, and oxidative modification of lipoic acid moieties on mitochondrial proteins. These data suggest that herbicide treatment placed a severe oxidative stress on mitochondria, whereas chilling and particularly drought were milder stresses. Detailed analysis of the soluble proteome of mitochondria by gel electrophoresis and mass spectrometry revealed differential degradation of key matrix enzymes during treatments with chilling being significantly more damaging than drought. Differential induction of heat shock proteins and specific losses of other proteins illustrated the diversity of response to these stresses at the protein level. Cross-species matching was required for mass spectrometry identification of nine proteins because only a limited number of pea cDNAs have been sequenced, and the full pea genome is not available. Blue-native separation of intact respiratory chain complexes revealed little if any change in response to environmental stresses. Together these data suggest that although many of the mo Cellular homeostasis can be disrupted by changes in the extracellular environment that uncouple biochemical pathways, which normally operate under a range of biophysical and chemical constraints. Such disruption is often accompanied by the undesirable accumulation of metabolic intermediates, and the most studied of these are reactive oxygen species (ROS) 1 produced by disruption of metabolism and electron transport chains. Much attention has focused on the impact of ROS during normal development and aging of cells and during perturbation of normal cellular life by disease, toxins, and the physical environment. Oxidative stress modification of proteins tends to increase with age in most organisms (1), but plants have recently been noted as an exception to the normal trend and are able to manage oxidative protein damage much more effectively during growth and later reproductive phases of life (2).Mitochondria form a focus for much oxidative stress research as not only are they the sites of oxygen consumption and a significant source of cellular ROS, but oxidative damage of the organelles perturbs the cell's energy supply required for repair mechanisms. Consequently the nature of oxidative damage to mitochondria is b...