ABSTRACT Changes in isolated frog rod outer segments, suspended in hypotonic solutions, have been examined by light and electron microscopy. Swelling of the disk occurs in hypotonic solutions. When one half or more NaCl is omitted from the Ringer solution used for suspending the rod outer segments, swelling is accompanied by the appearance of localized, irregular expansions projecting as buds from the disks. The axes of the buds tend to be in the plane of the disk, as can be seen in cross-sections of outer segments. In longitudinal sections of outer segments, the sectioned buds have profiles which were previously interpreted as vesicles. Attention is drawn to the properties of the disk edge, among which is a resistance to extension.