We show that in contrast to metaphase II oocytes, metaphase I oocytes cannot be activated by fusion with the zygote. Fusion of metaphase I oocytes with G2 zygotes was followed by premature chromosome condensation, with 60% of the hybrids becoming arrested at metaphase I, the remainder progressing and arresting at metaphase II. Hybrids of metaphase I oocytes and M-phase zygotes underwent accelerated maturation, but all arrested at metaphase II. In both cases the arrest could be overcome by treatment with the parthenogenetic activators ethanol and cycloheximide. We discuss these findings in relation to the possibility that the metaphase I oocyte contains cytostatic factor activity that is activated by its zygotic partner. Alternatively, the G2 zygote may provide an inhibitor of anaphase, normally never present in the metaphase I oocyte and which is absent from the M-phase zygote.