Abstract： AIM　To investigate the basis on which ectopic beats and focal atrial fibrillations originating from the pulmonary vein. METHODS　We studied 5 adult dog hearts. A block of tissue encompassing left atrials and pulmonary veins was dissected. Serial sections from this tissue were stained with hematoxylin and eosin, Masson's trichrome. Then slides were examined by light microscope. RESULTS　Cardiac muscles of left atriums extended onto pulmonary veins (myocardial sleeve). Myocardiums in the pulmonary veins course by various directions sometimes appeared internal circular and external longitudinal, sometimes appeared only longtitudinal or oblique. Pulmonary vein walls of dogs were composed of 3 layers: the tunica intima, tunica media and tunica adventitia. P-like cells scattered in the myocardium sleeve, and mainly distributed in the middle part of myocardium sleeve and almost were located in the superior pulmonary veins. No apparent morphological differences were found between myocardical cells in the pulmonary veins and myocardial cells in the left atriums. CONCLUSION　Ectopic beats from pulmonary veins triggering atrial fibrillation may be the results of P-like cells in the myocardial sleeves in the pulmonary veins.