The midbrain contains circuits that modulate locomotion. To delineate some of the involved regions, low-level stimulation (25 microA, 10 s train of 0.5 ms pulses at 50 Hz) was applied to the midbrain during locomotor stepping. Stepping was elicited in the anesthetized (pentobarbital, 40 mg/kg) rat by stimulating the hypothalamus with 0.5 ms pulses at 40 Hz at various currents. The rat was held in a stereotaxic apparatus such that locomotor stepping movements turned a wheel. Facilitation of locomotion was produced by stimulation in the anterior ventromedial midbrain and in the posterodorsal midbrain. When presented alone, such stimulation produced locomotion. Inhibition of locomotion was produced by stimulation of the superior colliculus (ventral layers) and the ventromedial midbrain. Additional inhibitory sites were found in the central gray and the lateral tegmentum. Inhibitory collicular stimulation, when presented alone, was characterized by the absence of any hindlimb response. Inhibitory ventromedial stimulation, when presented alone, frequently produced poststimulation locomotion and when presented with hypothalamic stimulation was characterized by postinhibitory increases in locomotion. These results indicate that: (1) the locomotor effects of stimulation in midbrain and hypothalamic sites can summate: (2) multiple locomotor suppressive systems are present in the midbrain and among them are a collicular system and a ventromedial system.