Summary (1) Changes during development in the levels of proteinases and peptidases were measured in brain homogenates. At all ages di- and tripeptidase levels were 7–15-fold higher than proteinase activity. (2) Cathepsin A and D and neutral proteinase activity first decreased (during the 5 days before birth) and then increased (primarily during the first 10 days after birth) in development. The total enzyme content per unit weight of brain did not change greatly after 10 days, although specific activity fell owing to an increase in protein in older animals. (3) The developmental pattern of activities of peptidases measured with Leu-Gly and Leu-Gly-Gly and of arylamidases measured with Arg- and Arg-Arg-β-naphyhylamides was similar to that of proteinases. Total and specific activities increased rapidly after birth; then total activity did not change and specific activity decreased. (4) The proteinase content of tissue fractions (nuclear and lysosomal-mitochondrial) similarly reached a maximal peak in the rapid growth phase of the brain. (5) The decrease of hydrolytic activity after 10 days of age seems to parallel a decrease in the rates of protein breakdown in vivo, showing parallel behavior with decreasing protein turnover. In contrast, during the first 10 days of life protein turnover and calculated rate of protein breakdown in vivo decrease while the level of hydrolytic enzymes increases.