During chicken growing, the m. soleus and m. pectoralis fibers become early 10-fold larger, the number of capillaries increase, they elongate and the blood velocity diminishes. A linear relation between PO2 and number of capillaries was revealed in the course of postnatal growth. The arterio-venous oxygen difference increased in the m. soleus' vascular bed and decreased in the m. pectoralis' one.