Locomotion is the result of a complex interplay between a centrally generated rhythmic pattern of activity and a host of feedback and feedforward modulatory signals that arise in pathways from the periphery and from the brain. In my presentation, I will discuss how the central structures that give rise to the major descending pathways in the cat may participate in ensuring that the basic locomotor rhythm, generated in the spinal cord, is adapted to the vagaries of the environment and the goals of the organism. I will concentrate on two structures and on two aspects of locomotor control. In the initial part of the presentation, I will concentrate on the role of the brainstem reticular formation in modifying the level of electromyographic (EMG) activity in flexor and extensor muscles of each of the four limbs of the cat. I will present data that suggest this structure is organised to produce coordinated modifications of the EMG activity in each of the four limbs of the cat. I will further suggest that the PMRF acts through the central pattern generator (CPG) in the spinal cord to ensure that the modifications of gait are appropriately incorporated into the base rhythm. Consideration of the anatomy and physiology of this pathway suggests that relatively simple descending signals may result in complex modifications of gait. In the second part of the presentation, I will concentrate on the role of the motor cortex in producing the fine control of limb trajectory and foot placement that is required in a normal, cluttered environment. I will show that the command signal descending from the motor cortex to the spinal cord contains detailed information about the modification of EMG activity that is required to step over an obstacle in one's path. I will further suggest that the final transformation of this command signal into the resultant pattern of EMG activity depends on the excitability of interneurones that either form part of, or that are influenced by, the CPG.