The ontogeny and distribution of immunoreactive gastric inhibitory polypeptide (IR-GIP) has been studied in the rat duodenum and jejunoileum between day 20 post-coitum and day 150 post-partum by radioimmunoassay following tissue extraction. In foetuses on day 20 of gestation, the content of IR-GIP was 2.0 +/- 0.2 ng/duodenum and 18 +/- 2.0 ng/jejuno-ileum. The IR-GIP content increased during the whole period investigated and a peak of concentration was observed in the first post-natal week in both the duodenum (860 +/- 150 ng/g) and jejunoileum (650 +/- 135 ng/g). In the duodenum, the main increase in IR-GIP content (ng per organ) occurred in the 3rd week of life and the same phenomenon was observed 2 weeks later in the jejuno-ileum. The highest GIP concentration was found in the duodenum and in the proximal jejunum, indicating that the upper small intestine is the major site of IR-GIP storage. It is concluded that the ontogeny of IR-GIP further supports the concept that this hormone is involved in the endocrine regulation of metabolism and suggests that this regulatory process takes place at an early stage of rat development.