Summary. The third (4-cell) and fourth (8-cell) cell cycles of early mouse development have been analysed in populations of blastomeres synchronized to the preceding cleavage division. DNA content was measured microdensitometrically. The entry of blastomeres into these cell cycles showed considerable heterogeneity both within and between individual embryos. This heterogeneity was greater in the fourth than in the third cell cycle. The component phases of the third cell cycle were estimated as G, = 1 h, S = 7 h, and G2 + M = 2–5 h, and those of the fourth cell cycle as G1 = 2 h, S = 7 h, and G2 + M = 1–3 h.