Using as an excitatory background the motor responses of a hind limb, elicited reflexly or by contralateral cortical stimulation, inhibition from the cortex was studied in cats, dogs and monkeys under dial, chloralose or urethane anesthesia. Bilateral inhibitory effects were obtained from either hemisphere in the 3 species. This inhibition was more readily obtained in the cat from the area frontalis agranularis than from the Betz cell area. Several lesions of the central nervous system were performed to determine the probable path of the inhibitory tracts in the cat. Evidence was obtained that the corpus callo-sum and the cerebellum are not involved and that the spinal path is mainly in the anterior columns. Crossing over was demonstrable below the midthoracic region. The independence of the excitatory and the inhibitory effects which may be evoked by stimulation of the cerebral cortex are discussed. The influence of the several anesthetics employed on motor and inhibitory effects from the cortex and on the spinal reflexes in the 3 species is described and discussed.