The development of the endoplasmic reticulum (ER) and the ultrastructural localization of glucose-6-phosphatase activity have been studied in the proximal jejunum and distal ileum during the postnatal period. One day after birth, the amount and the repartition of ER in the jejunal enterocytes are similar to that observed in postweaning period. In the following days an extensive proliferation of SER is noted in the supranuclear zone of the absorbing cells. From day 7 till postweaning period a gradual decrease of the amount of SER is observed and after weaning, the ultrastructure of the enterocytes is similar to that in the adult mouse enterocytes. At all time, a positive reaction for G-6-Pase activity is observed in the cisternae of the endoplasmic reticulum and in the nuclear envelope. In the distal ileum, the SER is poorly developed one day after birth. During the first two weeks, the ER increases but no extensive proliferation of SER can be noted as in the jejunum. The G-6-Pase activity can be visualized in the rough and smooth endoplasmic reticulum as well as in the nuclear envelope. It appears that the proliferation of SER could be interpreted as the morphologic expression of an increased G-6-Pase activity.