In curarized cats it was shown that stimulation of various central structures in the brain influences the size of an afferent volley evoked by a dorsal root stimulus and recorded from dorsal and ventral columns, midbrain, cerebellum and sensory cortex. Depression of the afferent test responses was obtained by stimulating the bulbar and midbrain reticular formation, the ventral part of the anterior vermis, the postcentral sensory cortex, the second somatic sensory area, the precentral motor cortex and the anterior part of the cingulate gyrus. So far, no increase in the afferent response, except as a rebound, has been obtained by central stimulation. The effect of the central stimulation upon the afferent responses was completely abolished by a moderate dose of anesthesia. Anesthesia also caused a marked increase in the control size of the afferent ventral column response. A similar effect was produced by a high transection of the spinal cord in curarized animals. It is concluded that synaptic afferent transmission in the spinal cord can be influenced in a physiological manner by descending pathways from certain structures in the brain. Furthermore, this sensory-regulating mechanism can evidently act in a tonic fashion.