The effects of long-term starvation on the activities of sucrase, lactase, and aminopeptidase, and on their respective mRNA were determined in the small intestine of thyroidectomized and sham-operated adult rats. Thyroidectomy reduced the protein loss at the level of the intestinal brush border membranes during starvation. Prolonged fasting caused a significant decrease in sucrase activity, but thyroidectomy partly prevented this effect. However, the amount of the corresponding mRNA dropped during long term starvation without incidence of thyroidectomy. Lactase activity in the brush border membranes was increased by starvation, and thyroidectomy caused a further elevation of the enzyme activity. Simultaneously, lactase mRNA content rose only slightly compared to the enzyme activity. Aminopeptidase activity and mRNA content decreased during starvation and thyroidectomy did not prevent this process. These results indicate that intestinal hydrolases respond non-coordinately to long-term food deprivation. In addition, the thyroid status of the animals has a direct influence on the adaptation of several brush border hydrolases to starvation. This suggests that the drop in plasma thyroid hormones during fasting allows a better maintenance of protein content and of hydrolase activities in the brush border membranes of the small intestine. These adaptive processes seemed to be partly controlled at a post-transcriptional level.