Following intraperitoneal inoculation with coxsackie B3 virus all weaning Syrian golden hamsters (60 animals) developed severe myocarditis. In the acute phase, light and electron microscopy revealed massive cellular infiltration and myocytolysis in the myocardium which were most prominent on the 5th day and least obvious in 3 weeks. In the necrotic myocytes, mitochondria contained moderately electron dense inclusions different from the calcified granules seen in the myocardium of mice with coxsackie B3 virus myocarditis. Macrophages ingested and digested necrotic cell debris, leaving moderate fibrosis but no calcification in the myocardium. Viruses were isolated from the myocardium on the 3rd to 9th day, and virus particles were seen in a necrotic cardiocyte on the 9th day. In the chronic phase, most animals developed no cardiomegaly; light microscopy revealed minimal myocardial fibrosis; electron microscopy showed various degenerative changes in some cardiocytes. A few animals (two animals) developed significant cardiomegaly with moderate to marked myocardial fibrosis in the 6th and 14th month. In one heart there was marked biventricular dilatation. In these animals with cardiomegaly, ultrastructural changes of the myocardial cells were similar to those described in congestive cardiomyopathy in man. Golden hamsters appear to be a unique model for a study of the possible relationship between viral myocarditis and idiopathic cardiomyopathy in man.