The ruffled border of osteoclasts in immature bones undergoing bone remodelling was three dimensionally examined with a scanning and a transmission electron microscope. The materials used the mandibular bodies obtained from newborn Wistar-strain rats. Scanning electron microscopic observations of the ruffled border were carried out by maceration and extraction of osteoclast cytoplasmic matrix with 0.1% osmium tetroxide. Multinuclear osteoclasts, which possessed a large number of mitochondria and endocytotic vacuoles, were found on the bone. They abutted on the bone surface by specialized membrane complex wellknown as the ruffled border and clear zone. The deepest portions of the membranous invaginations of the ruffled border viewed from the cytoplasmic side showed three kinds configurations : low and narrow ridges (70-110 nm in width), high and wide ridges (180-400 nm in width), and high and large bulges (300-400 nm in diameter). In this scene, hallows varying in size and direction between the ridges or the bulges were observed and appeared as cytoplasmic projections of the ruffled border. The ridges and bulges are probably coincident with the configurations of the proximal margins of deep membranous invaginations such as vacuolated swellings or finger-like protrusions seen in the ultrathin sections of the ruffled border. Small spherical protrusions corresponding to coated pits, from which coated vesicles are probably produced, were found at the summits of the high ridges and bulges, and thus coated vesicles are thought not to arise from the low and narrow ridges. The cytoplasm just adjacent to proximal margins of ruffled border included a large number of vacuoles and large phagosomes. The ridges and bulges are responsible for the production of such endocytotic vacuoles. These structural variations of invaginations of the ruffled border may reflect the transition of both osteoclastic function and working area.