Human skeletal muscle has been studied in vivo by 31P nuclear magnetic resonance. Young adult subjects were compared with healthy elderly subjects aged 70–80 years. Intracellular pH and concentrations of adenosine triphosphate (ATP), phosphocreatine and inorganic phosphate were measured. At rest, no differences were found between the two age groups. Aerobic dynamic exercise resulted in a similar reduction of phosphocreatine in both groups while the ATP concentration did not change. The pH fall in exercise was not significantly different in the elderly and younger subjects. The time to resynthesize ATP in the recovery period as measured by the time to replete phosphocreatine was also not different in the two groups. The results suggest that the aging process does not affect the metabolic ability of human skeletal muscle to respond to exercise and that changes found in muscle of the elderly are not due to alterations in energy productio