During the postnatal development of the rat submandibular gland, the origin of the membranes of the cytoplasmic vacuoles in terminal tubule cells was studied by light and electron microscopy. The terminal tubule cells before weaning contained the extensive endoplasmic reticulum, the small Golgi apparatus and abundant granules. Slight vacuolization was seen in the perinuclear area just before weaning, and after weaning the vacuoles rapidly increased in number. Acid phosphatase activity was localized in lysosomes and in newly formed vacuoles. The enzyme reaction product was also observed in the vesicles within the vacuoles. Using unbuffered osmium method, very intense staining was observed in the newly formed vacuoles as well as in the outer cisternae, the vesicles of Golgi apparatus and frequently in cavity between the double membranes of the endoplasmic reticulum. The newly formed vacuoles were a double membrane surrounding sacs which contained clearly recognizable cell organelles and/or secretory granules. Within the large vacuoles several organelles were present. Therefore, the cytoplasmic vacuoles in terminal tubule cells corresponded to crinophagy. These results suggest that the autophagic vacuoles in terminal tubule cells are derived from the endoplasmic reticulum and fused with lysosomes and then the inner membrane is gradually digested.