Objective　To detect the location of pathogens in myocardium using in situ RT-PCR technique in order to study the pathogenetic course of the myocardial lesion induced by CoxB3m virus infection in mice. Methods　(1) Thirty and fifty Balb/c mice were used respectively to establish the acute and chronic CoxB3m infected models, with another 25 healthy mice as the controls; (2) KS400 image analysis system (Germany) was used to measure the cardiac chamber area and the left ventricular wall thickness of the chronic infected mice and the controls; (3) CoxB3m virus in myocardial tissue was detected using in situ RT-PCR by direct incorporated technique which employed nucleotide labeling by anti-digoxin antibody and bonded with alkaline phosphatase (anti-dig-AKP method). Results　Picture analysis indicated that the left ventricular chamber area was enlarged and the left ventricular wall was thinner in the chronic repeated virus infected models than those of the controls. With in situ RT-PCR, positive signals for Coxsackie virus B3m RNA were detected not only in the myocardium of the acute Balb/c mice models but also in the myocardium of the chronic mice models. Conclusion　Coxsackie virus B3m is able to induce pathologic lesions by exhibiting positive CVB-RNA signals in both acute and chronic models in mice. In the chronic experimental models, the cardiac chamber is enlarged while the ventricular wall is thinned which demonstrates the association with persistent infection of Coxsackie virus B3m virus.