The DNA contents of normal R. pipiens embryos and R. pipiens female X R. sylvatica male embryos at various developmental stages were estimated by the method of Hoff-Jorgensen (1951). The DNA content of R. pipiens embryos remains constant from fertilization until near the end of segmentation, at which point it increases at a constant rate until shortly before muscular movements begin. The DNA content of hybrid embryos (which do not gastrulate) is identical with that of R. pipiens embryos until the latter reach the neural fold stage, at which point it does not increase further. The significance of these results is discussed briefly.