Intramuscular (m. quadriceps) contents of electrolytes, free amino acids and energy-rich phosphagens were determined in 12 patients (51--81 years of age) with moderate chronic obstructive lung disease. At the time of the study the patients were in their habitual condition. After an overnight rest, 8 patients showed hypoxia and five of these also hypercapnia. Compared to apparently healthy controls of similar age and studied under identical conditions, the patients had increased intramuscular concentrations of sodium, chloride and extracellular water, whereas magnesium was slightly reduced. ATP and phosphocreatine, as well as the ATP/ADP and phosphocreatine/total creatine ratios, were reduced. The ATP/ADP ratio correlated significantly to intracellular magnesium. Plasma amino acids were essentially similar in controls and patients but the intramuscular contents of essential amino acids and of the three branched-chain amino acids were slightly enhanced in the patients.