Relationships between the diameter of muscle afferent nerve fibers activated in a single volley of impulses and changes in the excitability of motor neurons supplying the contralateral hindlimb was found in unanesthetized spinal and decerebrate cats. Alterations in average size of monosynaptic reflexes evoked from muscles of the opposite limb were used to indicate excitability variations. Two effects were produced by a volley of Group I fibers in a muscle nerve on motor neurons innervating the corresponding muscle of the crossed limb: inhibition (with brief central delay) related to muscle spindle afferent fibers; a subsequent facilitation attributed to Golgi tendon organ fibers.