The aim of the present work was to investigate, in a more extensive way, the oxidative state and parameters related to energy metabolism of the heart tissue of rats using the model of adjuvant-induced arthritis. The latter is a model for the human arthritic disease. Measurements were done in the total tissue homogenate, isolated mitochondria and cytosolic fraction. The adjuvant-induced arthritis caused several modifications in the oxidative state of the heart which, in general, indicate an increased oxidative stress (+80% reactive oxygen species), protein damage (+53% protein carbonyls) and lipid damage (+63% peroxidation) in the whole tissue. The distribution of these changes over the various cell compartments was frequently unequal. For example, protein carbonyls were increased in the whole tissue and in the cytosol, but not in the mitochondria. No changes in GSH content of the whole tissue were found, but it was increased in the mitochondria (+33%) and decreased in the cytosol (-19%). The activity of succinate dehydrogenase was 77% stimulated by arthritis; the activities of glutamate dehydrogenase, isocitrate dehydrogenase and cytochrome c oxidase were diminished by 31, 25 and 35.3%, respectively. In spite of these alterations, no changes in the mitochondrial respiratory activity and in the efficiency of energy transduction were found. It can be concluded that the adjuvant-induced arthritis in rats causes oxidative damage to the heart with an unequal intracellular distribution. Compared to the liver and brain the modifications caused by arthritis in the heart are less pronounced on variables such as GSH levels and protein integrity. Possibly this occurs because the antioxidant system of the heart is less impaired by arthritis than that reported for the former tissues. Even so, the modifications caused by arthritis represent an imbalanced situation that probably contributes to the cardiac symptoms of the arthritis disease.