The molecular mechanisms by which mutations in the gene for Cu/Zn superoxide dismutase (SOD1) lead to the selective death of motor neurones in familial amyotrophic lateral sclerosis (FALS) remain incompletely understood. Previous evidence has indicated that mitochondrial abnormalities may develop during motor neurone injury, but several important questions remain unanswered. We have developed a cell culture model of FALS in which a motor neurone cell line (NSC34) has been stably transfected to express normal or mutant human SOD1 at levels approximating to those seen in the human disease. The aims of the study were to: (i) investigate whether morphological mitochondrial abnormalities occur at expression levels of mutant SOD1 close to physiological levels; and (ii) determine whether the presence of mutant SOD1 causes abnormalities of mitochondrial respiratory chain function and changes in cellular bioenergetic parameters in motor neuronal cells. Using this cellular model, we demonstrate that the presence of mutant SOD1 results in the development of abnormally swollen and pale staining mitochondria. These morphological changes are accompanied by biochemical abnormalities with specific decreases in the activities of complexes II and IV of the mitochondrial electron transfer chain. These same complexes are inhibited when control NSC34 cells are subjected to oxidative stress induced by serum withdrawal. The decrease in respiratory chain complex activity in the presence of mutant SOD1 was not accompanied by decreased expression of representative proteins present in these complexes. Motor neuronal cells expressing mutant SOD1 showed increased cell death when exposed to oxidative stress by serum withdrawal, whereas the presence of normal human SOD1 exerted a protective effect. Under basal, unstressed culture conditions, no change in the ATP : ADP ratio was observed in the presence of mutant SOD1. However, the mitochondrial changes associated with the presence of mutant SOD1 clearly had adverse cellular bioenergetic consequences as shown by increased cell death in the presence of pharmacological inhibition of the glycolytic pathway. We conclude that one important mechanism by which mutant SOD1 causes motor neurone injury involves inhibition of specific components of the mitochondrial electron transfer chain. Therapeutic measures aimed at protecting mitochondrial respiratory chain function may be useful in SOD1 related familial and possibly other forms of amyotrophic lateral sclerosis.