1. 1. Responses in 250 single alpha lumbar motoneurons were recorded during adequate stimulation of the labyrinthine and neck receptors, in decerebrated cats with deefferented hind limb. Adequate stimulation was performed by passive head movements in the three planes, such as ventro- and dorsiflexion, torsion around the longitudinal axis, and rotatory acceleration in the horizontal plane. 2. 2. Discharge changes were seen as response to head movements in about half of the recorded units; the rest showed no reaction. The extensor alpha motoneurons reacted to movements of the head mostly with brief discharge increase regardless of the direction of the head movement; during the period of maintained position slight frequency changes slowly occurred. 3. 3. After the neuronal responses were examined with intact neck and labyrinthine receptors, the neck afferents were eliminated by bilateral intradural section of the dorsal roots C 1-C 3 or a new method, the blockage of the afferent fibers by Pantocain injection into the intervertebral foramen. The facilitation of extensor alpha motoneurons previously observed during the movement was now abolished, and marked facilitation for the period of maintained head position appeared, graded for the different positions. In flexor neurons inhibition was the rule. These effects are due to activities originating in the vestibular organs. 4. 4. Definite rotatory acceleration in the horizontal plane produced facilitation in flexor alpha motoneurons during ipsilateral acceleration and inhibition during contralateral acceleration. Increase in discharge rate and latency were related to the amount of acceleration. Extensor alpha motoneurons showed opposite effects with flexor neurons, even when they are tested with the H-reflex. 5. 5. With the single unit analysis the antagonistic interaction of neck and labyrinthine activities upon alpha motoneurons can be shown. The neck receptors respond predominantly to head movements and they have an inhibitory influence upon the strong labyrinthine reflexes which arise in the otoliths during maintained head positions. These findings give a strong evidence that the so called tonic neck reflexes are not tonic but kinetic and have their receptors only in the small vertebral joints.