Background: To elucidate the hypothesis that gastrin and Cholecystokinin (CCK) are local growth factors for colorectal mucosa, we have examined the peptide gene expression in rat colon during development. Methods: Northern analysis, reverse transcription PCR, and sequence-specific radioimmunoassays were the essential methods. Results: High concentrations of gastrin and CCK messenger RNA were found in the fetal colon. At birth, gastrin and CCK mRNA's were both undetectable but increased subsequently towards adult life. The fetal colon contained 5.5 and 4.2 pmol/ g tissue gastrin and CCK, respectively. After birth, carboxyamidated gastrin disappeared from the colon, whereas the concentration of CCK remained at 1 pmol/g. Glycine-extended gastrin and CCK were also present in the fetal colon, but towards adult life they decreased below 0.2 pmol/g. In contrast, progastrin and proCCK were detectable at all ages. Conclusions: Rat colon expresses the gastrin and CCK genes throughout life. The posttranslational maturation of progastrin, however, ceases shortly after birth, indicating that gastrin may play a role in the developing colon. Whether CCK influences the development remains to be shown.