DNA polymerase activity was measured in mouse embryos at stages before implantation to determine whether it increases in proportion to the amount of DNA synthesis, as it does in populations of differentiated mammalian cells, or remains constant, as it does in early sea urchin embryos. Total enzyme activity was found to be relatively unchanged following fertilization and in the first few cleavage stages. However, between the 12- and 120-cell (blastocyst) stage, the amount of activity increased by several-fold. These results indicate that the relationship between amount of DNA polymerase activity and DNA synthesis in mouse embryos exhibits two phases: in the early cleavage phase it is similar to that in sea urchin embryos, whereas, in the blastocyst phase, it is similar to that in differentiated mammalian cells.