Capillary density, fiber cross-sectional area, capillary/fiber ratio and fiber composition were determined in gastrocnemius and diaphragm muscles of laboratory rats following five weeks at 350 Torr ambient pressure. Growth rates of hypoxic rats were lower than normoxic controls, but the periods of sacrifice were adjusted to achieve similar body masses for the two groups. Hypoxic rats had significantly higher hematocrits and heart masses, characteristic of acclimation to hypoxia. However, capillary/fiber ratios in gastrocnemius and diaphragm were similar for hypoxic and normoxic rats suggesting that hypoxia did not stimulate capillarity in either muscle. In diaphragm, but not gastrocnemius, diffusion distances were significantly shorter in hypoxic rats than in controls. We conclude that the differences in diffusion distances represent an important secondary effect of hypoxia on the diaphragm associated with changes in demands on ventilation in response to low oxygen pressures.