NASA is currently collecting extensive baseline electroencephalogram data under controlled conditions in a form suitable for mathematical analysis. Data are being taken from about 200 subjects in major national and overseas centers. It is intended that this study will assist in astronaut selection and monitoring in space. Studies on many effects of weightlessness on nervous functions require monitoring of the autonomic nervous system, including such autonomic effects as gastrointestinal activity, secretion, lacrimation, salivation, sweating, and the central control of respiration. Urinary estimations of catecholamines and 5-hydroxyindoleacetic acid would provide important data on autonomic system activity if collected in flight and compared with preflight and postflight controls. Major areas have been outlined in which prolonged weightlessness may be expected to interfere with performance, judgment, and, ultimately, chances of survival. These include cardiovascular, metabolic, central nervous, psychophysiological, and biorhythmic effects. They have been dealt with separately and in sequence, but have not been intended to be viewed as hierarchic. The relative scarcity of data necessarily precludes such an evaluation. Soviet experience with zero gravity and weightlessness has increased their emphasis on this space-flight factor and was an important topic at the May 1964 COSPAR meeting. Discussion of the postflight medical status of Bykovsky (5-day flight) and Tereshkova (3-day flight) revealed a concern for the significance of prolonged weightlessness and the presence of postflight physical debility and fatigue following Vostok flights 3 through 6. These changes persisted for several days. Among the physiological conditions singled out for mention were— (1) _Body fluids_— Cosmonauts have shown a postflight weight loss of 1.9 to 2.4 kg apparently resulting from a redistribution of body fluid in response to elimination of the hydrostatic pressure gradients caused by Earth gravity. There is the suggestion that this redistribution is complete within the first 24 hours of flight. Titov is reported to have been dehydrated alter his flight with early hemoconcentration. These findings directly support predictions made from ground-based research. (2) _Cardiovascular_— Postflight orthostatic tachycardia is reported for Titov as long as 23 hours after landing; at 48 hours there was significant residual intolerance to the upright posture. Cosmonauts have demonstrated a 20- to 35-percent increase in oxygen consumption during the standard postflight exercise test.