The evolution of ultimobranchial bodies in Holtzman rats during the first 64 weeks after birth was studied by reconstructing three-dimensional models from serial sections stained by the periodic acid-Schiff technique.Radio-autography with 125I was made to see if ultimobranchial cells and/or follicular cells lining the lumen of mixed follicles were able to iodinate proteins. The term ultimobranchial body designates herein an embryonic vesicular structure (derived from the third pharyngeal pouch) whose wall is made of stratified squamous epithelium. During the first week after birth, the vesicular ultimobranchial body elongates rapidly and becomes a canal or a duct. During the second week, cell desquamation brings about local dilatations in the lumen of these ducts; with further enlargement ultimobranchial follicles will appear. In one-day-old rats, mixed follicles are present. Only the follicular component of mixed follicles iodinates proteins as is shown by radioautography. The reconstructed models enlarge rapidly up to the 56th day after birth at which time their weight has increased nineteenfold. These same models show that the three morphological components of ultimobranchial parenchyma, namely ducts, follicles and mixed follicles, are in continuity within the thyroid parenchyma. The formation of new thyroid follicles after birth and the possiblility that the ultimobranchial parenchyma may function as an endocrine gland of holocrine type are discussed.