To study the genetical expression of lactase-phlorizin hydrolase in the jejunum and colon of developing rats we cloned the corresponding cDNA in a cDNA library constructed from the intestinal RNA of fetal rats. The structure of this 6.1 kb-long cDNA was similar to that of the cDNA of adult rabbits and humans, indicating that the enzyme was synthesized as a precursor organized into repeated homologous domains. The mRNA for lactase-phlorizin hydrolase accumulated at a constant level throughout the development in the jejunum despite the decline in lactase activity occurring at weaning. In the colon, the cDNA for lactase-phlorizin hydrolase hybridized to a transcript that was similar in size to the jejunal mRNA. This mRNA was transiently found during the first week after birth. Its rate of accumulation defined the level of lactase activity measured at this stage. We conclude that the expression of lactase-phlorizin hydrolase during the development is essentially regulated at the post-transcriptional level in the jejunum, in contrast to the colon where it is regulated at the pretranslational level.