Altered fluid equilibrium in buoyant states is accompanied by shifts in intracellular and extracellular electrolyte distribution, especially sodium and potassium. Evidence from recumbency studies indicates a strong correlation between loss of erect posture or weight bearing and excretion of calcium stores in bone. A bone X-ray densitometry method has been developed by Mack, at Texas Woman’s University, for accurately determining the loss of bone mineral (±2 percent accuracy) in humans and animals. The heel bone and spine are X-rayed using a calibrated aluminum wedge as a standard. This technique will be used for preflight and postflight analysis of the primate being flown in the 30-day biosatellite. Comparative appraisal of bone mineral behavior in astronauts participating in the Gemini and Apollo programs will be invaluable for future flight missions. Bed rest and immobilization studies by Mack have shown loss of skeletal mineral and increased calcium in the urine and excreta. Four bed-rest studies, each extending for 2 weeks, compared different levels of calcium intake. Four men were used in each study and served as their own controls during extended ambulatory periods. During 2-week periods, up to 10 percent of calcium mineral was lost from the heel bone. Calcium was also determined in the urine and feces. In other studies, isometric exercises reduced loss of bone mineral during bed rest. Excretion of calcium in the urine is accompanied by risk of its deposition as calculi or "kidney stones" in the urinary tract. Currently, changes in calcium metabolism resulting from weightlessness over periods up to 2 weeks is not considered a hazard requiring precautionary measures. Flights in excess of 2 weeks, however, constitute a problem serious enough to warrant study on the 11-day orbital flights and the 30-day biosatellite primate mission. Therapeutic immobilization, post-poliomyelitis immobility, and experimental restraint in normal subjects lead to a negative calcium balance, with hypercalciuria. Central Nervous System Functions in Weightlessness The wide range of individual tolerances to the disturbing effects of vestibular stimulation has emphasized the importance of this factor in astronaut selection. At the same time, vestibular functions must be considered jointly with visual task performance, since both have special significance for such maneuvers as vehicle docking. Vestibular function in the weightless state remains almost completely unknown. Limited evidence from animal and manned space flights suggests that head turning, resulting from vestibular stimulation, may seriously interfere with visuomotor performance, but that susceptibility to these disturbances is significantly different between individuals and that partial adaptation occurs relatively quickly.