Ostriches have been filmed running fast in their natural habitat. A female ostrich has been dissected and the principal bones, muscles and tendons in a leg have been measured. It is calculated that stresses up to 240 kN m−2 and 40 MN m−2, respectively, act in the digital flexor muscles and their tendons during running. Tensile and compressive stresses up to about 70MNm−2 and 110 MNm−2 act in the tibiotarsus. A large proportion of the energy which would otherwise be required for running is probably saved by elastic storage in tendons. Comparisons are made with the legs of flying birds and of antelopes.