An experiment was designed to evaluate the vestibular contribution to the stabilization of upright stance in normals and in two patients with loss of vestibular function. A forward or backward displacement of a load (2 kg) by a torque motor attached to the subject induced opposing movements in the head and trunk. The small linear acceleration of the head in space of about 0.1 g was followed, with a latency of 50-65 ms, by EMG responses in the tibialis anterior and rectus femoris (backward acceleration) or gastrocnemius muscles (forward acceleration). These responses were absent in patients with a vestibular deficit. It is suggested that the observed EMG responses are due to fast acting vestibulospinal reflexes involved in the regulation of upright stance. For comparable head accelerations the integrated EMG responses induced by the vestibulospinal mechanism are about ten times smaller than those induced by spinal stretch reflexes during displacement of the feet. Vestibulospinal reflexes would appear, therefore, to play only a minor role in the compensation of stumbling.