Programmed cell death, or apoptosis, occurs during the development of many tissues and organs in almost all multicellular organisms. Although apoptosis of salivary gland cells has been demonstrated in several pathological conditions, the role of apoptosis in the postnatal development of the salivary glands is unknown. We have studied the development of the rat submandibular gland (SMG) during its transition from the perinatal stage to the mature adult stage. Terminal tubule or Type I cells, which synthesize the secretory protein SMG-C, are prominent in the perinatal acini and are believed to form the intercalated ducts of the adult gland. Between 25 days and 30 days after birth, the number of Type I cells and their SMG-C immunoreactivity markedly decreased. Apoptotic cells in association with the developing intercalated ducts were labeled with the Terminal Deoxyribonucleotidyl Transferase-Mediated dUTP Nick End Labeling (TUNEL) method. Between 25 and 40 days of age, from 50 to 80% of the apoptotic cells in cryostat sections of the SMG were closely associated with the intercalated ducts. Electron microscopy showed that the Type I cells became vacuolated, their secretory granules were reduced in size and number, and the amount of rough endoplasmic reticulum was decreased. Cellular debris resembling apoptotic bodies was phagocytosed by macrophages and adjacent intercalated duct cells. These observations suggest that the loss of Type I cells and reduction of SMG-C immunoreactivity during development of the intercalated ducts of the adult rat SMG is due, at least in part, to apoptosis.