The central portion of the medial head of the gastrocnemius of control (normoxic and normothermic), hypoxia-, cold-, and cold plus hypoxia-acclimated guinea pigs was analyzed for capillary supply and fiber composition to elucidate changes in capillarity induced by environmental stresses. The muscle was cut at midbelly, frozen, sectioned, and stained for myosin ATPase. Fiber cross-sectional areas; percentages of slow-twitch oxidative (SO), fast-twitch oxidative-glycolytic (FOG), and fast-twitch glycolytic (FG) fibers; and numbers of capillaries around each fiber type were measured. Growth rates of all four guinea pig groups were similar. Capillarity was not affected by acclimation to hypoxia. Cold and cold plus hypoxia acclimation led to increased numbers of capillaries around the fiber in all three fiber types. In addition, significant increases in the percentage of FOG fibers and concomitant decreases in the percentage of FG fibers compared to controls were found in cold and in cold plus hypoxia indicating that a transformation of fiber type from FG to FOG had occurred. The increase in FOGs at the expense of the FGs did not occur in the guinea pigs grown in a hypoxic environment. The increased total capillarity in those muscles studied was the result of more capillaries around all fiber types and was not due to simple transformation of fibers.