Human subjects were vertically oscillated in the dark over a frequency range of 1-0.02 HZ at peak-to-peak acceleration amplitudes of 0.2-0.6 g, using both a vertical movement simulator and real flight. Subjective tracking of the movement was unreliable and showed no systematic dependence of phase upon frequency. In contrast, reflex oculomotor response, although weak, clearly demonstrated progressive and substantial phase lag with increasing frequency. The similarity of this characteristic to that previously obtained from brainstem neural responses in cat suggests the oculomotor response was primarily of a vestibular origin. The in-flight studies demonstrated that head movement in a changing linear accelerative field, especially at 0.1 Ha, is highly provocative of motion sickness despite subthreshold angular movement of the aircraft.