Changes of distribution of immunoreactive gastrin (IRG) and molecular forms of intra-tissue gastrin in the growth course of rats were examined. The results obtained were as follows; 1) IRG concentrations and G cell counts were extremely low in the fetal antral mucosa. However, a gradual increase of the above was observed during the neonatal suckling period, accompanied by a rapid increase at the commencement of feeding. 2) IRG concentrations in the duodenal mucosa of the fetal and neonatal period were markedly higher than that of adult rat. 3) Jejunal IRG concentration was negligible in the fetal period. The value obtained from the postnatal rats was equal to or higher than that of adult rat. 4) The major form of antral gastrin was G-17 throughout the fetal and adult age. No qualitative change of antral IRG in the growth course of rats was seen. 5) The major form of the duodeno-jejunal mucosa was G-17 in the fetal period and thereafter G-34 increased gradually in the growth course of rats. These results suggested that, (1) suckling and feeding appeared to be a trigger of the production and release of antral gastrin in the growth course of rats. (2) In the initial stage of the growth, G cells distributed in the duodeno-jejunal mucosa as well as in the antral mucosa may participated in the production and release mechanism of gastrin.