The ventricular surface of the subfornical organ of the rabbit's brain was studied with scanning and transmission electron microscopic techniques. The ependymal covering was found to consist of hexagonal cells with convex apical surfaces. From the center of each cellular surface a single kinocilium up to 6 μm in length protrudes into the liquor. It is usually covered with secretory material having the shape of pearlstrings. The surface aspect of the subfornical organ suggests secretion into the liquor by emptying of giant vacuoles which originate below the ependyma in nerve cells, move towards the surface, develop pressure while flattening their ependymal cover and finally erupt, leaving collapsed ependyma- and/or nerve cells bag on the surface of the organ. A second mechanism of more granular secretion by ependymal cells appears possible.