Brefeldin A, a fungal metabolite which disrupts protein traffic, provokes indirect activation of cdc2 protein kinase in Xenopus oocytes. Cdc2 protein kinase activation was judged by MPF (M-phase factor) transfer activity, histone H1 kinase activity, and phosphorylation in vivo of the guanine-nucleotide exchange complex EF-1βγδ. Oocytes resumed complete meiosis upon brefeldin A treatment. Cdc2 protein kinase, MAP kinase, cyclin B, MPF, and protein synthesis changes were all comparable in brefeldin A-treated oocytes and in progesterone-induced oocytes. ED 50 for brefeldin A was 0.6 μ M. Brefeldin A activation of cdc2 protein kinase occurs with a long time course. Simultaneous treatment of the oocytes at a subthreshold concentration of 1 n M progesterone and 30 μ M brefeldin A considerably shortened the kinetics of maturation. Brefeldin A induction of maturation was sensitive to drugs that act on cAMP metabolism. ID 50 for IBMX was 0.1 m M, compared to 1 m M for progesterone-treated oocytes. Brefeldin A inhibited protein traffic in oocytes as determined from protein export experiments. ID 50 was between 0.1 and 1 μ M. Our results give new insights into the possible mechanism of induction of meiotic maturation and further demonstrate that brefeldin A acts on cell cycle regulatory elements.