1. Muscle capillarity was examined in chicks exposed to normobaric hypoxia with and without increased workload. 2. Capillarity was higher in the lateral head of the gastrocnemius, but not in the other muscles examined. Increased workload did not contribute to the effects of hypoxia. 3. Thus, hypoxia served as a stimulus for the development of new capillaries in muscle containing twitch fibers, but not in muscle containing tonic fibers.