SummaryThe cytosol peptidase activities hydrolyzing glycyl-L-leucine and L-leucyl-glycyl-glycine as well as the activities of the brush border peptidases (oligoaminopeptidase, aminopeptidase A, γ-glutamyl-transpeptidase, dipeptidyl-aminopeptidase IV, and carboxypeptidase) are present in rat fetuses during the early differentiation of the intestine (17th to 19th days of fetal life); they increase then at a different rate, reaching a maximum at various times, in the second and third wk after birth, and then decrease to the adult values during the first month of postnatal life. Only the oligoaminopeptidase activity increases steadily after birth, reaching maximal activity in the last decade of the first month.In human fetuses aged between 8 and 22 wk, the γ-glutamyl-transpeptidase was the only brush border peptidase found to be higher than in adults and children. On the other hand, the amino-peptidase A is remarkably reduced. The dipeptidylaminopeptidase IV and the carboxypeptidase are already at adult level in the youngest fetuses, and the oligoaminopeptidase increases during the period of fetal life studied; at the end of this period, the enzyme activity does not differ from the values found in children and adults.The small intestine of the term and preterm newborn should, therefore be able, with some possible exceptions, to efficiently digest peptides.