The ventricular endocardium of 20 rats and five dogs was examined with a scanning electron microscope after careful washing with phosphate-buffer, glutaraldehyde fixation and freeze-drying. Endothelial cells formed a continuous sheet lining the ventricle in both species. Individual cells were identifiable by a central bulge corresponding to the nucleus and were separated by a polyhedral pattern of intercellular lines. The intimal surfaces of these cells were relatively smooth, although sometimes transverse ridges, small pseudopod-like projections or indentations suggestive of macropinocytosis were present. Endothelial cells were relatively easily lost in preparation and shrinkage artefacts were also observed. These observations are compared with other scanning electron microscopic studies of endothelium and are discussed with particular reference to the effects of preparative procedures.