Cell cycles in early amphibian embryos are characterized by the absence of G1 and G2 phases. The simple cycle of S phase and mitosis does show similarities with other systems, particularly in the presence of cytoplasmic components advancing nuclei into DNA synthesis and mitosis. Maturation-promoting factor induces nuclear envelope breakdown and subsequent chromosome condensation. Cytoplasmic factors appear during maturation which are capable of inducing DNA synthesis, and arrest of the nuclear division cycle in metaphase (cytostatic factor). The timing of appearance of these activities is considered and their relationship in integrating DNA synthesis during early cleavage is discussed.