The development of 31P-nuclear magnetic resonance (NMR) has enabled direct and non-invasive measurements of muscle metabolism. Serial measurements of the phosphocreatine/inorganic phosphate (PCr/Pi) ratio, which is closely related to the adenosine triphosphate/adenosine diphosphate (ATP/ADP) ratio and pH during and after forearm exercise were performed in 11 patients with chronic lung disease (CLD), nine patients with chronic heart failure (CHF) and eight control subjects. As compared with control subjects, the PCr/Pi ratio in the patients with CLD or CHF was lower during the recovery period and significantly lower at three and 4 min exercise. The pH values after exercise were lower in patients with CLD or CHF compared to control subjects. The PCr/Pi ratio at 4 min after exercise in the patients with CLD or CHF did not correlate with parameters of cardiac function or arterial and mixed venous oxygen tension. The arterial oxygen content and output in patients with CLD and CHF were significantly lower than that of control subjects. Nutritional parameters were not statistically different among the three groups. These observations suggest that metabolic abnormalities may be present in the skeletal muscles of patients with CLD and CHF that are not due to under-nutrition. These may result from reduced arterial oxygen output and, partially, from physical detraining.