Neurons in the brainstem implicated in the initiation of locomotion include glutamatergic, noradrenergic (NA), dopaminergic (DA), and serotonergic (5-HT) neurons giving rise to descending tracts. Glutamate antagonists block mesencephalic locomotor region-induced and spontaneous locomotion, and glutamatergic agonists induce locomotion in spinal animals. NA and 5-HT inputs to the spinal cord originate in the brainstem, while the descending dopaminergic pathway originates in the hypothalamus. Agonists acting at NA, DA or 5-HT receptors facilitate or induce locomotion in spinal animals. 5-HT neurons located in the parapyramidal region (PPR) produce locomotion when stimulated in the isolated neonatal rat brainstem-spinal cord preparation, and they constitute the first anatomically discrete group of spinally-projecting neurons demonstrated to be involved in the initiation of locomotion in mammals. Neurons in the PPR are activated during treadmill locomotion in adult rats. Locomotion evoked from the PPR is mediated by 5-HT(7) and 5-HT(2A) receptors, and 5-HT(7) antagonists block locomotion in cat, rat and mouse preparations, but have little effect in mice lacking 5-HT(7) receptors. 5-HT induced activity in 5-HT(7) knockout mice is rhythmic, but coordination among flexor and extensor motor nuclei and left and right sides of the spinal cord is disrupted. In the adult wild-type mouse, 5-HT(7) receptor antagonists impair locomotion, producing patterns of activity resembling those induced by 5-HT in 5-HT(7) knockout mice. 5-HT(7) receptor antagonists have a reduced effect on locomotion in adult 5-HT(7) receptor knockout mice. We conclude that the PPR is the source of a descending 5-HT command pathway that activates the CPG via 5-HT(7) and 5-HT(2A) receptors. Further experiments are necessary to define the putative glutamatergic, DA, and NA command pathways.