This study shows the distribution of the messenger RNA for lactase-phlorizin hydrolase during postnatal development and along the longitudinal axis of the rat small intestine. At birth, this messenger RNA was present along the whole length of small intestine, and its concentration remained elevated during the suckling period despite the concomitant decrease in enzyme activity. At weaning, the amount of lactase messenger RNA dropped specifically in the distal ileum. This decrease in lactase messenger RNA was initiated at the ileocecal junction, progressed gradually towards the jejunum, and followed the decrease in lactase activity several days later. Starvation and refeeding were also found to cause modifications of lactase activity and messenger RNA expression that were prominent in the distal part of small intestine. These data support that posttranscriptional and pretranslational levels of regulation are required to define the spatial and temporal expression of lactase in the rat small intestine.