The total amount of DNA and the protein/DNA ratio have been measured in the kidneys, heart, liver, and gastrocnemius muscles of 56 human fetuses and newborn infants of 13-42 weeks' gestational age. The total amount of DNA in each organ approximately doubled every week up to the 25th; thereafter the rate of increase was slower. The protein/DNA ratio in each organ increased rapidly in the last 10 weeks of intrauterine life. Before 30 weeks there was some increase in the protein/DNA ratio in the kidneys, heart, and gastrocnemius muscles, but no significant change in the liver. In 5 small-for-dates infants the protein/DNA ratio was normal, whereas the total DNA tended to be low.