It is generally known that the anuran stomach begins to express pepsinogens (Pg) during metamorphosis. To clarify the mechanisms of differentiation of Pg-producing cells, we examined immunohistochemically the epithelial transformation from larval to adult form in Xenopus laevis stomach at the cellular level. At the beginning of metamorphic climax, concomitantly with the modification of the basement membrane, apoptotic cells labelled by TUNEL suddenly increased in number in the entire epithelium except for the primordia of adult epithelial cells in the basal region of larval glands. Subsequently, with the development of connective tissue, the adult epithelial cells actively proliferated and replaced the larval cells from the basal to the luminal region. Following the start of morphogenesis of adult glands, Pg-producing cells became differentiated in newly formed adult glands, but not in the adult surface epithelium. We then developed an organ culture system and examined effects of thyroid hormone (TH) on the differentiation of Pg-producing cells in X. laevis stomach in vitro. In the presence of TH, just as in spontaneous metamorphosis, Pg-producing cells differentiated from the adult epithelial primordia after the apoptosis of larval epithelial cells. In contrast, in the absence of TH, neither apoptotic larval cells no Pg-producing cells were detected. Therefore, we conclude that TH triggers organ-autonomously the entire process leading to the differentiation of Pg-producing cells in X. laevis stomach. In addition, the strict localization of Pg-producing cells in the adult glands both in vivo and in vitro suggests the correlation between the differentiation of Pg-producing cells and morphogenesis of the glands surrounded by the developed connective tissue.