Orbital spaceflight exposes astronauts to an environment in which gravity is reduced to negligible magnitudes of 10-3to 10-6G. Upon insertion into earth orbit, the abrupt loss of the constant linear acceleration provided by gravity removes the otolith stimulus for vestibular sensation of vertical orientation constantly present on Earth. Since the central nervous system (CNS) assesses spatial orientation by simultaneously interpreting sensory inputs from the vestibular, visual, and proprioceptive systems, loss of the otolith-mediated vertical reference input results in an incorrect estimation of spatial orientation, which, in turn, causes a degradation in movement control. Overtime, however, the CNS adapts to the loss of gravitational signals. Upon return to Earth, the vertical reference provided by gravitational stimulation of the otolith organ reappears. As a result, a period of CNS readaptation must occur upon return to terrestrial environment. Among the physiological changes observed during the postflight CNS readaptation period is a disruption of postural equilibrium control. Using a dynamic posturography system (modified NeuroCom EquiTest), 16 astronauts were tested at 60, 30, and 10 days preflight and retested at 1 to 5 hours, and 8 days postflight. All astronauts tested demonstrated decreased postural stability immediately upon return to Earth. The most dramatic increases in postural sway occurred during those sensory conditions in which both the visual and proprioceptive feedback information used for postural control were altered by the dynamic posturography system, requiring reliance primarily upon vestibular function for control of upright stance. Less marked but statistically significant increases in sway were observed under those conditions in which visual and foot support surface inputs alone were altered. Recovery to preflight postural stability levels, which were well within normal clinical limits, occurred in all subjects by 8 days postflight. The results demonstrate that 5- to 10-day losses of gravitational inputs from the vestibular otoliths to the CNS during orbital flight severely disrupt the otolith mediated sensorimotor interactions required for normal terrestrial postural control. Readaptation to normal postural control was complete in all astronaut subjects within 8 days postflight