The ontogeny of 23 postcranial skeletal dimensions, mechanical properties of the third metatarsal and the femur, and the isometric contractile properties of the gastrocnemius muscle were measured in a complete postnatal growth series of the highly cursorial hare Lepus californicus. Newborn hares are handicapped by short limbs, relatively weak muscles and bone tissue, and shorter muscular contractile distances. However, during growth these deficiencies are rapidly overcome. Hind limb length and gastrocnemius contractile distance undergo positive allometry, scaling as (body mass)0·39 and (body mass)0·41, respectively. The mechanical advantage of the gastrocnemius lever system experiences negative allometry (a(body mass)-0·12), giving young hares larger advantage around their joints. This, combined with initial positive allometry of gastrocnemius contractile force (α(body mass) 1·23) and subsequent negative allometry of contractile force (α(body mass)0·61), results in the production of relatively greater propulsive forces transmitted from the foot to the ground in juveniles than in adults. In addition, the second moment of area of the metatarsal is relatively large in younger animals, and scales in such a way that during the first half of postnatal growth mechanical similarity is maintained between the breaking moment of the bone (α(body mass)1·50) and the moment imposed on the bone by the gastrocnemius (a(body mass)l·54). These ontogenetic changes contribute to the development of a locomotor system which is effective at escape by the time the young hare is only 20% of adult body size, and must forage independently. Differences between the ontogenetic allometry of Lepus, the interspecific allometry of mammals and the ontogenetic allometry of reptiles are noted and considered in a phylogenetic context.