The Syrian hamster polymyopathy is a hereditary disease, transmitted by an autosomal recessive gene, involving the heart and the entire musculature. The chronology of the pathologic events in the myocardium and skeletal muscle has been investigated in UM-X7.1 myopathic hamsters aged 0-250 days. A phasic pattern in the progression of the disease process was evident. Microscopic necrotic changes in the heart were visible prior to or at 50 days of age with increasing severity until 100 days of age and subsidence thereafter. More than 50% of the animals died before 250 days of age with signs of cardiac failure. The intensity and extent of myocardial calcific changes together with scar formation were determinant factors in curtailing the survival of animals. Changes in serum creatine kinase (CK) activity followed a phasic pattern similar to the progression of the myopathic disease. Because of the disparity of disease manifestations between the different myopathic hamster lines, it is essential to consider the time course of the heart and skeletal muscle microscopic changes when evaluating the severity of the hamster polymyopathy.