In the context of energetics related to a pendular model, the mechanical power (W) and ‘pendular motion efficiency’ (PME) were determined during walking of the subjects who consist of 37 healthy elderly women (65-85 years) and 21 young women (18-25 years) . Using a force plate, the potential and kinetic energies of the body's centre of mass were measured at various constant speeds. Walking speeds were selected and controlled by a newly devised pace-maker. PME, which is equivalent to ‘% recovery’ by Cavagna (1976), indicates a sort of efficiency in transforming potential energy into kinetic energy and vice versa. The external power to accelerate the body (Wext ), which is thought to be supplied by muscles, increased with walking speed, and the rate of increase in Wext tended to be greater in the elderly than in the young subjects. It was noted that the maximal PME values at the optimum speed in both age groups were comparable, but PME values in the elderly decreased more markedly than in the young subjects as walking speed deviated from the optimum. This fact suggests that an adaptability to different walking speeds reduced in the elderly population.