Serum gastrin concentration became became detectable in rats at one week after birth (56 pg/ml), and a rapid rise to 198 pg/ml was seen at two weeks after birth. Gradual decreases were seen again at three weeks and four weeks, and thereafter it attained the adult level of 132 +/- 17.6 pg/ml. In regards to the gastrin-like immunoreactivity (GLI) in the gastrointestinal mucosa during the fetal period, the values were extremely low even in the antral mucosa. However, the GLI showed an increase during the neonatal suckling period, and was accompanied by a remarkable increase at the commencement of feeding. This GLI coincided with so-called little gastrin, and while an increase accompanied by the development of growth was seen, no qualitative changes were evident. GLI values were high in the duodenal mucosa followed by the antrum, and positive evidence was also seen in the corpus of the stomach and jejunum. High values of GLI in the mucosae were gradually seen at two weeks after birth coinciding with the rise in serum gastrin levels, and the significance of the increases was discussed from the anlge of the trophic effect of gastrin.