The effect of glucocorticoids and thyroid hormones on lactase expression was investigated along the small intestine of rats. In sucklings thyroxine injections promoted a precocious drop of enzyme activity but not of mRNA. Hydrocortisone did neither modify lactase activity nor its mRNA expression. In adults decreasing the amount of thyroid hormones led to a slight and reversible increase of the lactase mRNA content whereas lactase activity rised more dramatically. Thus, thyroid hormones in contrast to corticoids, are involved in the posttranscriptional control of lactase during the suckling period. Yet, none of these hormones might induced the modification of the longitudinal distribution of the lactase mRNA that occurred at weaning.