Xenopus oocyte maturation is a model system for studying the control of cell proliferation and the regulation of the cell cycle. Addition of progesterone or insulin to oocytes releases a G2 block and stimulates progression through meiosis to an unfertilized egg. The release of the G2 block is a consequence of a decrease in cAMP mediated entirely or in part by an inhibition of adenylate cyclase. The mechanism of cyclase inhibition involves a membrane steroid receptor controlling the rate of guanine nucleotide exchange. Subsequent events include an increase in intracellular pH and the phosphorylation of ribosomal protein S6. The latter event may play a role in translational control of maturation. Late events in maturation involve the appearance of the maturation-promoting factor (MPF), a cytoplasmic protein responsible for causing nuclear envelope breakdown, chromosome condensation, and spindle formation. MPF oscillates in meiotic and mitotic cell cycles. The events caused by MPF can now be obtained in crude extracts with retention of cell cycle control by calcium, providing a framework for rapid progress in characterizing MPF and its regulation.