The descending pathways from the brainstem locomotor areas were investigated by utilizing reversible cooling (to block synaptic or fiber transmission) and irreversible subtotal lesions of the brainstem or spinal cord (C2-C3 level). Experiments were conducted on decerebrate cats induced to walk on a treadmill by electrical stimulation of the brainstem. Locomotion produced by stimulation of the mesencephalic locomotor region (MLR) was not abolished by caudal brainstem lesions that isolated the lateral tegmentum or by extended rostral/caudal dorsal hemisections of the spinal cord. These results demonstrate that the MLR does not require a pathway projecting through the lateral tegmentum of the brainstem or the dorsal half of the spinal cord, as previously suggested (Mori et al., 1977, 1978b; Shik and Yagodnitsyn, 1978; Shik, 1983). Rather, the results indicate that the descending pathway originating from the MLR projects through the medial reticular formation (MedRF) and the ventral half of the spinal cord. Locomotion produced by stimulation of the pontomedullary locomotor region (PLR) was blocked by reversible cooling of either the MedRF or the ventrolateral funiculus of the spinal cord. In some cases, locomotion could be produced by stimulation of the PLR following extended dorsal hemisections of the spinal cord. These results demonstrate that the PLR can also produce locomotion by activation of cells in the MedRF that project caudally through the ventral half of the spinal cord. Stimulation of the PLR could also elicit locomotion following its surgical isolation from the MedRF of the brainstem. Furthermore, lesions of the dorsal spinal cord resulted in the loss of PLR-evoked locomotion in some, but not all, cases. Thus, an alternative projection of the PLR through the dorsal half of the spinal cord (Kazennikov et al., 1980, 1983a,b; Shik, 1983) cannot be ruled out. Overall, these results demonstrate that the PLR is not an essential component of the motor pathway originating from the MLR. The organizational scheme of “brainstem locomotor regions” is discussed in the context of recent information demonstrating a link between the sensory component of the trigeminal system and locomotor pathways (Noga et al., 1988).