The metabolism of the skeletal muscles during exercise and recovery was investigated using phosphocreatinine (PCr) and inorganic phosphate (Pi) in patients with chronic heart failure. PCr/Pi ratio, which is closely related to the ATP/ADP ratio, and the pH were measured by 31P-magnetic resonance spectroscopy (31P-MRS) during and after forearm exercise in 9 patients with chronic heart failure, 11 patients with chronic lung disease, and 8 normal subjects. Exercise and recovery scans were recorded every minute for 4 min. The PCr/Pi ratio in patients was lower during the recovery period and significantly lower 3 and 4 min after exercise than in normal subjects. The pH values after exercise were lower in patients, although not significantly. The PCr/Pi ratio 4 min after exercise in patients was not correlated with parameters of cardiac function or arterial and mixed venous oxygen tension. Nutritional parameters did not vary statistically among the groups. Metabolic abnormalities may be present in the skeletal muscles of patients' group, which are not due to undernutrition, possibly as a result of exercise deconditioning and probably a shift in fiber distribution (type I decreases, type IIb increases) and a decrease in oxidative capacity.