The effects of caloric stimulation of the labyrinth on the soleus motor pool have been investigated using the Achilles tendon reflex, the H reflex, and the vibratory inhibition of the H reflex. The excitability of the myotatic reflex are is augmented from the beginning of irrigation and remains so until the end of nystagmus. The effects are bilateral and roughly symmetrical. The observed effects are due to simultaneous activation of non-specific, probably reticular, and specific, doubtless vestibular, , mechanisms. The Achilles tendon reflex is more facilitated than the H reflex, and the vibratory inhibition of the H reflex is not modified following irrigation of the ear canal. These results indicate a simultaneous facilitation of alpha and gamma extensor motoneurones but gamma effects clearly predominate. This interpretation is in good agreement with the findings of the animal neurophysiology. The vibratory inhibition argues against a modification of presynaptic inhibition. Before interpreting this fact as different from animal mechanisms, one must take into account the particular experimental conditions realized in man: the stimulus is complex and interactions between various nervous structures are possible.