Abstract Certain aspects of the mechanisms mediating auditory-induced spinal motor activity have been examined by recording within the brain stem and from spinal motoneurons at both cervicothoracic and lumbosacral levels. Single auditory click stimulation elicits, in lightly strychninized cats, motor responses appearing along both sides of the spinal cord. In the competition for access to the final common path, the evoked descending acoustic volley of impulses is readily blocked by prior dorsal-root stimulation. Since the ventral-root responses recorded contralaterally to the side of auditory stimulation have the same appearance, respond identically to higher frequency stimulation, and interact with dorsal-root responses in an identical way as do the ipsilateral ventral root responses, it is assumed that the acousticospinal mechanisms on the two sides of the spinal cord are mirror images of each other elicited from a common brain-stem neuronal pool. Decerebration and cerebellectomy do not interfere with the transmission of this acousticospinal reflex activity. Auditory projections to the brain-stem reticular formation are evidently sufficient to maintain the pattern of the descending reflexes. Partial sections of the spinal cord demonstrate that the descending connections necessary for the transmission of auditory induced activity are part of a diffusely projecting spinal system. Bilateral motoneuron discharges following unilateral acoustic stimulation are ensured by an abundance of functional crossings in the brain stem and along the extent of the spinal cord.