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.
