We have studied the effect of maturation-promoting factor (MPF) on embryonic nuclei during the early cleavage stage of Xenopus laevis development. When protein synthesis is inhibited by cycloheximide during this stage, the embryonic cell cycle arrests in an artificially produced G2 phase-like state, after completion of one additional round of DNA synthesis. Approximately 100 nuclei can be arrested in a common cytoplasm if cytokinesis is first inhibited by cytochalasin B. Within 5 min after injection of MPF into such embryos, the nuclear envelope surrounding each nucleus disperses, as determined histologically or by immunofluorescent staining of the nuclear lamina with antilamin antiserum. The breakdown of the nuclear envelope occurs at levels of MPF comparable to or slightly lower than those required for oocyte maturation. Amplification of MPF activity, however, does not occur in the arrested egg as it does in the oocyte. These results suggest that MPF can act to advance interphase nuclei into the first events of mitosis and show that the nuclear lamina responds rapidly to MPF.