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1. Proteolytic (substrate nitrocasein), tripeptidase (substrate glycylglycylgly-cine) and aminopeptidase (substrate leucyl-beta-naphthylamide) activities were studied in homogenates of jejunal and ileal mucosa of 7-, 10-, 14-, 21-, 35- and 60-day-old rats. 2. Proteolytic activity was practically the same in jejunum of 7-, 10-, 14- and 21-day-old rats, but after day 21 a significant increase was observed. The activity of the ileum changed very little during postnatal development and was always higher than that of the jejunum. 3. Tripeptidase activity was low in the jejunum of 7- and 14-day-old rats, an increase was observed between day 14 and 21, but later no substantial changes were found. There were no changes in the ileum. The activity in the jejunum of 7- and 14-day-old rats was lower than in the ileum, but later the jejunum was more active than the ileum. 4. Aminopeptidase activity had a similar developmental pattern to tripeptidase activity. A low activity was found in the jejunum of 7- and 14-day-old rats, the maximum was in 21-day-old rats and then a decrease was observed, though values for 60-day-old rats were still higher than for 7- and 10-day-old rats. The activity in the ileum was practically the same in all age groups studied except in 14- and 21-day-old rats, where a transient peak was observed.

β-Glucuronidase activity in the homogenates of the mucosa of the small intestine is higher in the distal than in the proximal third of the intestine between the 4th and 17th postnatal day in rats. Activity falls proximally up to day 20, while distally, activity rises from birth up to day 15 and then falls rapidly up to day 20 (20×). Starting from day 20, activity is the same proximally and distally. β-Glucuronidase (expressed as changes of total activity per third of the small intestine) changes during the postnatal development very little in the proximal part; the greatest changes occur in the distal part.Adrenalectomy, performed on day 15, inhibits the normal decrease in activity seen in the distal part on day 19.

During the postnatal development of mice, intestinal β-galactosidase activity (0.001 M o-nitrophenyl β-D-galactoside as substrate) is always higher at pH 3.5 than at pH 5.5. In the jejunum a steady decrease of activity during postnatal development is evident, whereas in the ileum the activity increases temporarily and later decreases. The changes in rabbits are more complicated. At pH 5.5 there is a steady decrease of β-galactosidase activity in the ileum and jejunum, but at pH 3.5 there is a steady decrease in activity in jejunum only; in ileum, first an increase is observed and then a decrease. In guinea pigs the activity in newborn animals is relatively very low, and only a small decrease during postnatal development is found. During postnatal development, the activity decreases less at pH 3.5 than at pH 5.5. In contrast to mice and rabbits, guinea pigs showed no substantial differences between jejunum and ileum in all the age groups studied.The results are discussed from the point of view of significance of differences between the jejunum and the ileum, and the existence of several types of β-galactosidase activity.