Morphological and biochemical studies were performed on the soleus muscles of rats receiving a daily intraperitoneal injection of 50 mg chloroquine chloride (CQ) per kilogram of body weight. Light microscopy showed mild to moderate variations in fiber size, numerous dense membranous bodies and vacuoles. The vacuoles were ringed by material that was intensely basophilic in hematoxylin-eosin preparations, resembling rimmed vacuoles in the muscle fibers of distal myopathy. Segmental degradation or necrosis was often observed. The 3H-leucine uptake by myofibrillar and soluble sarcoplasmic fractions in CQ-treated muscles was the same as in the controls. The significant increases in lysosomal (cathepsin B & L, B and D) proteases and thiol protease inhibitor occurred in the earlier stages of CQ-induced myopathy, when hardly any autophagic vacuoles or dense bodies were observable by light microscopy. We conclude that the over-development of autophagic vacuoles and the significant increases in lysosomal protease activity in muscle tissues may be important in the development of the focal degradation and necrosis of CQ-treated muscles.