1. 1. Trehalase, sucrase-isomaltase and maltase-glucoamylase are three integral glycoproteins of the brush border membranes of the enterocytes. On the basis of a comparative study on α-glycosidase activities (sucrase, isomaltase, maltase, glucoamylase and trehalase) associated to these glycoproteins during neonatal development, mammals could be basically divided into three groups. 2. 2. In rodents and rabbit α-glycosidase activities are low or undetectable during the suckling period and increase to adult levels during the weaning period. In cat, dog and the primates examined, α-glycosidase activities are well or fully developed at birth. 3. 3. In ruminants and pinnipedia α-glycosidases are low or absent throughout life. 4. 4. During the suckling period of rat, mouse and rabbit, glucocorticoids trigger a premature and dramatic increase of all α-glycosidases. 5. 5. On the contrary, α-glycosidases development during the weaning period appears to be independnt of glucocorticoids. Neither hypophysectomy nor adrenalectomy prevent the development of α-glycosidases; only the rate of increase is reduced. 6. 6. Transplantations of intestinal isografts either in adult or suckling animal, have shown that (1) no systemic factor inhibits the expression of α-glycosidase, (2) α-glycosidases induction is neither triggered by luminal alimentary substances, nor by hormones, (3) α-glycosidase development is controlled by an intrinsic ontogenic program. 7. 7. The use of an antiglucocorticoid failed to inhibit the spontaneous development of α-glycosidase activities. 8. 8. The increase of maltase and sucrase activities triggered by glucocorticoids is associated with an increase of the concentration of two glycoproteins in the microvillous membrane; sucrase-isomaltase and maltase-glucoamylase. 9. 9. After administration of glucocorticoids the increase of maltase, sucrase and trehalase is strongly inhibited by actinomycin-D and the increase of sucrase activity is associated with a parallel increase of sucrase-isomaltase mRNA. Transcription is most likely the primary site of control of α-glycosidase biosynthesis. 10. 10. In the crypt cells, α-glycosidases biosynthesis appears to be triggered by a receptor-mediated glucocorticoid interaction. 11. 11. The enterocytes synthesize more α-glycosidase molecules as they travel to the tip of the villi. 12. 12. The simultaneous, biosynthesis of sucrase-isomaltase and maltase-glucoamylase triggered by glucocorticoids, as well as their simultaneous normal development suggest that they may be subjected to related control mechanisms. 13. 13. It is suggested that sucrase-isomaltase and maltase-glucoamylase might have arisen by several cycles of partial gene duplication of an ancestor gene coding for a single site maltase-isomaltase; subsequent mutation would have transformed isomaltase into sucrase or glucoamylase.