Functional morphologists have traditionally regarded cost of locomotion as an important influence on the design of locomotor structures. If cost of locomotion is an important constraint in the natural selection of these structures, it should be possible to show that animals differing in limb morphology also differ in their locomotor costs. In previous experiments on three species of cursorial mammals differing considerably in limb structure, no such differences were detected. Since the factors that determine the rate of energy consumption of a running animal are not well understood, we felt that the effect of limb morphology on cost could best be examined in a system in which only the inertial properties of limbs were varied while other factors remained constant. Consequently, we have measured changes in the rate of energy consumption of running human subjects produced by artificial alterations in limb inertial properties. Other variables that might influence cost have been controlled. We found that the cost of adding a given mass to the limbs is significantly greater than adding it to the centre of mass and that this effect becomes more pronounced as the limb loads are moved distally. Thus a clear effect of limb mass and its distribution on cost of locomotion has been demonstrated.