Acute mid-thoracic spinal cord transection eliminates hindlimb air-stepping in neonatal rats suspended in harnesses and administered L-DOPA. Because spinal cord transection eliminates all descending inputs to the hindlimb locomotor circuits, this experiment determined if coadministration of L-DOPA and quipazine (serotonin receptor agonist) would induce hindlimb air-stepping in rat pups 24 hr after transection. Hindlimb steps of spinally transected pups that received L-DOPA or quipazine alone were infrequent and slow; hindlimb steps induced by L-DOPA + quipazine occurred more frequently and were faster than those elicited by either drug alone. These findings suggest that catecholaminergic and serotonergic systems both contribute to hindlimb stepping.