Hypoxia may be one of the factors underlying muscle dysfunction during ageing and chronic lung and heart failure. Here we tested the hypothesis that chronic hypoxia per se affects contractile properties of single fibres of the soleus and diaphragm muscle. To do this, the force-velocity relationship, rate of force redevelopment and calcium sensitivity of single skinned fibres from normoxic rats and rats exposed to 4 weeks of hypobaric hypoxia (410 mmHg) were investigated. The reduction in maximal force (P(0)) after hypoxia (p=0.031) was more pronounced in type IIa than type I fibres and was mainly attributable to a reduction in fibre cross-sectional area (p=0.044). In type IIa fibres this was aggravated by a reduction in specific tension (p=0.001). The maximal velocity of shortening (V (max)) and shape of the force velocity relation (a/P(0)), however, did not differ between normoxic and hypoxic muscle fibres and the reduction in maximal power of hypoxic fibres (p=0.012) was mainly due to a reduction in P(0). In conclusion, chronic hypoxia causes muscle fibre dysfunction which is not only due to a loss of muscle mass, but also to a diminished force generating capacity of the remaining contractile material. These effects are similar in the soleus and diaphragm muscle, but more pronounced in type IIa than I fibres.