The fine structure of the biomembrane of the sinus node cells in the dog wasstudied with the replicas of freezecleaved specimens.The intramembranous particles.(IMP)in the PF and EF faces of the plasma mcmbrane of the sinus node cells,were present.The density of the IMP on the PF faces was more dense than those ofEF faces,In general,the shape of the IMP of the plasma membrane(PF and EFfaces)exhibited spherical or spheroidal.The IMP of PF faces may be divided into,three groups,i.e.the larger,medium and small size.Numerous endocytotic vesicles.found on the PF faces appeared as circular depressions and as small walled craterson EF faces.No regular array of the vesicles had been observed on both faces,TheIMP of the mitochondrial cristae were more dense than those of the depression.TheIMP of PF face of the nuclear membrane were observed.Their size and shapepresent a uniform appearance.There were many nuclear pores in the surface of thenuclear membrance.In the cytoplasm,longitudinal and transverse sections of the myofibrils werefound.On the transverse section,the myofilaments of A and I band werepresent.The intercalated disk was observed.The gap junctions present between thesinus node cells were proved by Masson-Pevet et al in the rabbits but were notfound in our specimens.