A force platform has been used to obtain records of the forces exerted on the ground by running quail. These records, with cinematograph film taken simultaneously, have been used to determine the fluctuations of kinetic and potential energy occurring during a step. This gives a first estimate of the energy required for running. Graphs of force against length changes during a step have been calculated for the major leg muscles, making certain assumptions. These graphs provide a second estimate of the energy required for running, and also show the stresses and strains which probably occur in the muscles. The bending moments required to break the major limb bones have been determined, and compared with the bending moments calculated to occur in running. A mathematical discussion provides estimates of the energy cost of a range of techniques of running.