The vestibular system was activated by galvanic electrical stimulation in 19 normal subjects. With the head turned to one side so that the stimulating anode was on the posterior mastoid process, stimulation caused standing subjects to sway backwards in the sagittal plane. Electromyography showed bilateral activation of erector spinae, gluteus maximus, biceps femoris, soleus and intrinsic foot (toe flexor) muscles. When head direction or electrode polarity was reversed so that the anode was anterior, all those muscles became less active and the subjects swayed forwards. With the head facing forward, stimulation caused sideways sway in the coronal plane, towards the anode, with excitation of the erector spinae on the anode side and reduced activity on the cathode side. The limb muscles were activated on the side opposite the anode and showed complex responses on the anode side. Responses were detectable in the erectores spinae muscles in sitting subjects. No responses in limb muscles were detected in the sitting posture. Subject responses in erector spinae recorded at L3/L4 had latencies from 59 to 110 ms, using a 2 mA stimulus. Latencies in lower limb muscles were longer. The results suggest a role for the vestibular system and descending brain stem motor pathways to the erectores spinae muscles in the control of postural orientation of the back when sitting and standing. The conduction velocity in the motor pathway was estimated to be 13 +/- 10 m s(-1) (mean +/- S.D., n = 12 subjects).