To understand what processes affect the cell-cycle timing of mitotic events in early cleavage cycles of sea urchin embryos, a study was made on the effects of (a) reducing protein synthesis with emetine and (b) DNA replication with aphidicolin, on the timing of nuclear envelope breakdown, anaphase onset and cytokinesis. When protein synthesis was slightly inhibited by administration of emetine, the delay in the mitotic events increased, with an increase in the delay in accumulation of proteins up to the levels to which cells must synthesize the proteins to execute the cleavage. This indicated that protein synthesis affects the timing of mitotic events. The delay in cleavage cycles caused by a slight inhibition of DNA replication with aphidicolin was in proportion to the concentration of aphidicolin administered, suggesting that DNA replication also affects the timing of mitotic events. Furthermore, it was confirmed that accumulation of the proteins to the levels required for execution of the first cleavage precedes completion of DNA replication as a requirement for execution of the first cleavage. These results imply the existence of process(es) affected by protein synthesis that are included in a feedback control system which prevents the initiation of mitosis until after the completion of DNA replication; it is the characteristic of a cell-cycle control system that has been predicted theoretically.