In mouse eggs arrested at meiotic metaphase II, the increase in intracellular calcium that results from fertilisation induces nuclear formation in both newly ovulated and older eggs. In contrast, the calcium increase that results from exposure to the calcium ionophore A23187 induces nuclear formation in older, but not young, newly ovulated eggs. When treated with the microtubule inhibitor colcemid, and fertilised, young eggs remained at metaphase, but many older eggs formed nuclei, although older eggs treated with colcemid and A23187 remained at metaphase. However, young A23187-treated eggs, young colcemid-treated fertilised eggs, and older colcemid- and A23187-treated eggs, formed nuclei when treated, in addition, with the protein synthesis inhibitor cycloheximide, or the protein kinase inhibitor 6-dimethylaminopurine (6-DMAP). The possibility is discussed that metaphase in newly matured eggs and microtubule-depleted eggs may be maintained by similar mechanisms involving short-lived phosphorylated proteins.