In mitochondrial encephalomyopathies, impairment of the electron transfer chain may lead to overproduction of reduced oxygen species because oxygen consumption is decreased. Whether heat shock proteins (Hsp) are induced or not in mitochondria against oxidative stress is questionable. Muscle ragged-red fibres are the histological hallmark of most respiratory chain deficiencies in humans. They exhibit abnormal mitochondria which accumulate mainly under their sarcolemma. Within these fibres, immunolabelling demonstrated strong expression of mitochondrial manganese-dependent superoxide dismutase and a lack of expression of mitochondrial Hsp60 within the subsarcolemmal spaces. In contrast, Hsp60 was overexpressed within the intermyofibrillar mitochondria. These findings suggest enhanced generation and dismutation of superoxide anions and that processing and integration of imported precursor proteins is impaired within the subsarcolemmal mitochondrial aggregates of ragged-red fibres, whereas protein import and assembly may still be efficient in the intermyofibrillar mitochondria of these fibres.