The level of cyclin B-associated cdc2 kinase, a component of maturation promoting factor (MPF), is known to be high during metaphase of the meiotic maturation of oocytes. The time-related action of gonadotropin-releasing hormones (GnRH) on histone H1 kinase activity (known to reflect cdc2 kinase activity) was investigated in vitro in follicle-enclosed goldfish oocytes. Germinal vesicle breakdown (GVBD) and testosterone production were also investigated in the same follicle-enclosed goldfish oocytes to determine the temporal relationship between GnRH-induced histone H1 kinase activity and the reinitiation of meiosis and steroidogenesis. Treatments with gonadotropin (GTH) or GnRH stimulated the histone H1 kinase activity to the same maximum level. However, sGnRH- and cGnRH-II-induced histone H1 kinase activity were significantly higher compared with controls after 2 hours of treatment, whereas the GTH-induced increase became significantly higher after 6-8 hours of incubation. Overall, the results demonstrate a close temporal relationship between GVBD response and histone H1 kinase activity induced by GTH and sGnRH-cGnRH-II.