In Experiment 1, rats of 5, 10, 15, and 20 days of age were injected with L-DOPA to elicit stepping and videotaped while suspended in air and in warm water. Inter- and intralimb coordination were similar for L-DOPA-induced air-stepping and swimming at all ages. Other parameters of locomotion, such as step length and step rate, varied by substrate. In Experiment 2, pups of 5 to 20 days of age received an injection of L-DOPA or the vehicle and were videotaped while freely swimming. L-DOPA-induced free swimming was more stereotyped and "automatonlike" than that of vehicle-injected animals. Through 15 days of age, all pups swam using all four limbs, but L-DOPA induced stricter synchrony of movement in diagonal limbs than was used by pups injected with the vehicle. At postnatal Day 20, vehicle-injected pups adopted the adult swimming pattern in which only the hindlimbs were used, whereas L-DOPA-injected pups continued to use the immature pattern, diagonal progression with all four limbs. These results suggest that L-DOPA-induced air-stepping and swimming are manifestations of the same central mechanisms and that parameters of L-DOPA-induced stepping are modified by the sensory and/or biomechanical effects of the substrate. Although the free swimming of L-DOPA-injected pups was kinematically similar to vehicle-treated pups, L-DOPA retarded transition to the adult swimming pattern.