Unanesthetized decorticate cats walked or ran normally on a treadmill either spontaneously or during electrical stimulation of the subthalamic "locomotor" region. The respiratory response usually preceded the locomotor response and increased in proportion to locomotor activity despite control or ablation of respiratory feedback mechanisms. Respiration increased similarly in paralyzed animals during fictive locomotion despite the absence of muscular contraction or movement. Hypothalamic command signals are thus primarily responsible for the proportional driving of locomotion and respiration during exercise.