Transcellular oxygen flux in skeletal muscle fibres was modelled mathematically. In eels at the same environmental temperature (15 degrees C), changes in muscle structure associated with increased levels of activity elevated mean fibre Po2 by 30% to 5.2 kPa, despite greater fibre radius and Vo2, due to more capillaries and intracellular lipid. The latter results in a 68% increase in oxygen permeability ([symbol: see text]o2). While cold acclimation of striped bass (5 vs. 25 degrees C) led to a modest (12%) fibre hypertrophy, Vo2 fell proportionately more (by 60%). A 50% increase in capillary supply again aids oxygen flux, while the presence of intracellular lipid effectively reverses the cold-induced decrease in [symbol: see text]o2. The combined effect is to increase mean fibre Po2 from 1.9 to 4.6 kPa and minimum Po2 from 0.57 to 4.2 kPa, respectively. These data suggest little selection pressure exists to alter fibre composition in order to increase peripheral oxygen transport, while the magnitude of change in intracellular Po2 is likely in excess of that required to maintain locomotory activity. Hence, there may be some other factor than Po2 regulating structural reorganisation of muscle fine structure.