In this study we analyzed the ontogeny and location of gastric TRH in human fetuses, preterm and term newborns, and adults. TRH immunoreactive cells were found in the antrum towards the bottom of developing glands and double immunostaining demonstrated that this neuropeptide is coexpressed with gastrin in the same cell (G-cell). In the youngest fetuses studied (12 weeks) G cells were few and contained both gastrin and TRH. They increased in number during development and were most abundant between 26 and 36 weeks of gestation. These morphological data correlated with total immunoreactive TRH content extracted from the whole stomachs of six fetuses and two preterm infants. On the contrary G cells containing both hormones were decreased in the newborn at term and not identified in the adult whereas those containing only gastrin were numerous in both. The TRH extracted was indistinguishable from synthetic TRH using chromatographic, radioimmunologic, and enzymatic criteria. As has already been reported, TRH was found in insulin-containing cells of the islets of Langerhans in the pancreas of our fetuses and newborns. These cells presented a similar development pattern to the gastric G cells.