Xenopus oocytes and embryos undergo two major maternally controlled cell-cycle transitions: oocyte maturation and the mid-blastula transition (MBT). During maturation, the essential order of events in the cell cycle is perturbed in that the M phases of Meiosis I and II occur consecutively without an intervening S phase. Use of U0126, a new potent inhibitor of MAPK kinase (MEK), shows that MAPK activation is essential to inhibit the anaphase-promoting complex and cyclin B degradation at the MI/MII transition. If MAPK is inactivated, cyclin B is degraded, S phase commences and meiotic spindles do not form. These events are restored in U0126-treated oocytes by a constitutively active form of the protein kinase p90Rsk. Thus all actions of MAPK during maturation are mediated solely by activation of p90Rsk. At the MBT, commencing with the 13th cleavage division, there are profound changes in the cell cycle. MBT events such as maternal cyclin E degradation and sensitivity to apoptosis are regulated by a developmental timer insensitive to inhibition of DNA, RNA or protein synthesis. Other events, such as zygotic transcription and the DNA replication checkpoint, are controlled by the nuclear:cytoplasmic ratio. Lengthening of the cell cycle at the MBT is caused by increased Tyr15 phosphorylation of Cdc2 resulting from degradation of the maternal phosphatase Cdc25A and continued expression of maternal Wee1. Ionizing radiation causes activation of a checkpoint mediating apoptosis when administered before but not after the MBT. Resistance to apoptosis is associated with increased p27Xic1, the relative fraction of Bcl-2 or Bax in pro- versus anti-apoptotic complexes, and the activity of the protein kinase Akt.