The development of free walking was studied in rats between postnatal days 10 and 20. Spontaneous quadruped walking with the ventral surface of the body off the floor was first observed at postnatal day 11. Locomotion remained clumsy and invariably slow during the next few days, but a rapid transformation into the mature pattern of locomotion occurred around postnatal day 15. This transformation involved changes in quantitative parameters of locomotion as well as a change in the movement pattern of the hindlimb. A swimming-like movement characterized by abduction, rotation and hyperextension of the paw was replaced by the digitigrade adult pattern without marked rotation. Joint angle trajectories of the major joints during the step cycle changed considerably during the transitional period. The results, which are to serve as a framework for ongoing research into the effects of early undernutrition and movement restriction upon motor function, are discussed in the perspective of developmental changes in the nervous and musculoskeletal system.