Prior to the Mercury program, extensive efforts were undertaken to find ways of minimizing the mass of in-flight food systems. Such efforts were directed to the use of dehydrated, energy-dense foodstuffs and to the possibilities of nutrient recyclization. As the space program became a reality, nutritional scientists were more concerned with the mechanics of food consumption in weightless flight and with the problems of structuring convention foods in a way that would facilitate their use in weightlessness. It soon became clear that there was no substantial impediment to normal gastrointestinal function in flight, and attention was shifted to more subtle metabolic phenomenons. It became apparent that slight changes occurred in skeletal density, muscle mass, and overall body composition. Recognition of these changes led to extensive ground-based simulation studies and carefully designed in-flight experiments. Data are presented on the requirements for metabolic energy in flight and on the losses that have been observed in the major elemental constituents of the body. It is concluded that convincing evidence is not yet avialable on the ability of man to adapt to long-term weightless flight. Although his nutritional requirements are qualitatively similar during flight, the sophisticated manipulation of nutrient profiles shows promise of counteracting some of the deteriorative processes that are known to occur.