To evaluate the influence of age, DNA synthesis and brush border hydrolase activities were determined in mouse kidneys maturing in vivo and in serum-free organ culture. 2. DNA synthesis decreased with advancing age. 3. The protein content and leucylnaphthylamidase, maltase, trehalase, alkaline phosphatase and gamma-glutamyltransferase activities increased with aging. 4. The differences due to age were reproduced in kidneys maturing in culture. 5. These results show that age has a significant effect on the parameters determined, but apparently has no influence on the viability of the kidney explants in culture.