Fibre type composition in the soleus muscle was studied in second generation male and female Sprague-Dawley rats bred at 460 Torr (hypobaric hypoxia). In control male and female rats (bred at sea level atmospheric pressure) the percentage of slow twitch oxidative (SO) fibres increased, and the percentage of fast twitch oxidative glycolytic (FOG) fibres decreased over time. In male hypoxic rats fibre type composition did not change. However, in female hypoxic rats the percentage of SO fibres increased, and that of FOG fibres decreased over time. The female hypoxic rats had a higher percentage of FOG fibres and a lower percentage of SO fibres than age-matched control rats. These findings indicate that hypoxia-acclimatised rats have a relatively high percentage of FOG fibres, and that this is due to inhibition of the fibre type shift from FOG to SO, which occurs during normal development. Furthermore, there is a sex-related difference in the change in fibre type composition in hypoxia-acclimatised rats.