An effort was made to perform complete metabolic balance studies of 2 astronauts during a 10-day pre-flight control phase, 14 days of orbital space flight (NASA Gemini-VII), and 4 days of post-flight recovery phase, measuring dietary intakes and excretions of calcium, magnesium, phosphate, sulfate, nitrogen, sodium, potassium and chloride. In addition, urinary excretions of 17-hydroxycorticosteroids, aldosterone and catecholamines were measured in the same subjects. Technical or engineering constraints on biomedical observations during the flight prevented optimal performance during the in-flight phase, resulting in variations in dietary control (except for calcium, which was reasonably constant) and in losses of urine samples. Considerable interindividual variability was demonstrated in all experimental indices measured. In one man, significant increases in urinary calcium occurred during the second week of flight, and persisted during the recovery phase; calcium balance became less positive in flight in both subjects. Urinary phosphate excretion increased substantially in flight in both subjects despite reduction in phosphate intake. Urinary nitrogen and sulfate excretion decreased in flight but less than would be expected from the reduction in intake. Patterns of excretion of magnesium, sodium, potassium and chloride were different for each subject and could in part be correlated with changes in adrenocortical steroid production. The principal hormonal change was a striking decrease during flight in the urinary excretion of 17-hydroxycorticosteroids. Dermal losses of calcium, magnesium, sulfate, nitrogen and phosphate were insignificant during all 3 phases.