The ontogeny of intestinal CRT, AGAT and GAMT was investigated in foetuses, newborn, suckling, weaning and adult rats. In the colon, CRT mediates creatine transport because it was Na(+)- and Cl(-) dependent and inhibited by creatine and GPA. In addition, Northern assays showed two CRT transcripts (2.7-kb and 4.2-kb) and the in situ hybridisation revealed that CRT mRNA is restricted to the colon epithelial cells. The immunohistochemistry revealed that CRT protein was at the apical membrane of colon epithelia. Maturation decreased colonic CRT activity to undetectable levels and increased CRT mRNA abundance. Western assays revealed 57-, 65-, 80- and 116-kDa polypeptides at the intestinal apical membrane. The abundance of the 65-, 80- and 116-kDa polypeptides decreased with age, and that of 57-kDa was only observed in adult rats. The small and large intestine express AGAT and GAMT mRNAs. Maturation decreased AGAT mRNA abundance without affecting that of GAMT. For comparison, renal AGAT mRNA levels were measured and they were increased with age. The study reports for the first time that: i) the apical membrane of rat colon have an active CRT, ii) development down-regulates CRT activity via post-transcriptional mechanism(s), iii) the intestine might synthesize creatine and iv) intestinal and renal creatine synthesis is ontogenically regulated at the level of AGAT gene expression.