Blood capillary and muscle fiber densities were measured in histological sections of heart and skeletal muscles of the fossorial mole rat (Spalax ehrenbergi) and compared with those of the white rat. Average capillary densities in the masseter, gastrocnemius, rectus abdominis, and heart of the mole rat are 860, 750, 300, and 2,340 per mm² as compared with 490, 190, 190, and 1,420, respectively, in the white rat. The corresponding fiber densities are 680, 650, 540, and 3,430; and 510, 230, 280, and 2,280 per mm², respectively. The differences still exist when corrected for difference in body mass. Individual fiber area of the mole rat's muscles is one-half that of the rat. Erythrocyte diameter is 6.3 μm for the mole rat and 7.0 μm for the rat in accordance with the capillary diameter. The calculated tissue diffusion distance in the mole rat, 70% that of the rat, is regarded as an adaptation to fossoriality.