Internal surface structures of the auricular and ventricular chambers were studied with scanning electron microscopy. We observed vascular connections between the ventricular chamber and the coronary vessels, including cords and papillary muscles covering the lining of the endocardium. The endothelial cells covering the inner surface of the endocardium appeared to be functionally active with many microvilli similar to pinocytic microprojections of other endothelial cells. The pinocytic microprojections ranged between approximately 100 and 300 Å in diameter. The epicardium contained mainly bundles of collagenous fibers