Summary.1. In decerebrate or nembutalized cats whose pyramidal tracts were stimulated at the medullary level the typical response in leads from the dorsal surface of the lumbar part of the spinal cord was a recruiting slow negative deflection and a subsequent positivity. The response was not influenced by a transverse section of the medulla caudal to the point stimulated, sparing only the pyramids.2. A similar response could be elicited by stimulation of the pericruciate area in the cerebral cortex. When this area was sucked out and the pyramids were stimulated after an interval allowing the cortico‐spinal fibres to degenerate, no response was obtained in leads from the lumbar cord dorsum.3. In conditioning experiments the afferent cord dorsum response to low threshold stimulation of a cutaneous nerve in the hind leg (the N1, deflection) was used as the test response. The N1, deflection decreased in amplitude when superimposed on the slow negative wave elicited from the contralateral pyramid while it was not influenced following ipsilateral pyramidal stimulation. In an analogous way a depression of the N1, deflection was obtained when the pericruciate area was stimulated. When the slow negative wave was not elicitable due to degeneration of the corticospinal fibres stimulation of the pyramid did not influence the N1, deflection.4. The results indicate that the pyramidal tract mediates a cortico‐spinal inhibitory influence on the primary afferent relay in the spinal cord.