A study was made of the response of peripheral vestibular neurons in the squirrel monkey to head vibration and air-borne sound in the frequency range from 50-4 00 Hz. Responses were measured in terms of the phase locking of discharge and changes in firing rate. The lowest phase-locking thresholds for vibration were -70 to -80 dB re 1 g, and median values in the most sensitive frequency range (200-400 Hz) were -20 to -40 dB re 1 g; the minimum and median thresholds for sound were 76 and 120-130 dB SPL, respectively. Rate-change thresholds were 10-30 dB above phase-locking thresholds. The squirrel monkey sacculus has no special sensitivity to vibration in comparison with the other vestibular end-organs; the median phase-locking threshold to sound of saccular neurons exceeded 100 dB SPL. Irregularly discharging neurons are more sensitive than regularly discharging units. Evidence is presented that the response to intense sound involves a hair-cell mechanism.