Although high amounts of cyclin B1 mRNA are present in bovine oocytes arrested at the germinal vesicle (GV) stage, the protein is not detectable. Furthermore, there is a depletion of the stored cyclin B1 mRNA in the oocyte as follicular growth progresses. To assess the effect of follicular growth on the accumulation of M-phase promoting factor (MPF) components, mRNA and protein levels of cyclin B1 and p34 cdc2 were measured in GV oocytes collected from diverse follicle size groups (Ͻ2 mm, 3-5 mm, and Ͼ6 mm). Because oocytes collected from very small follicles have high levels of cyclin B1 mRNA, the onset of its accumulation in the oocytes was evaluated by in situ hybridization of fetal ovaries. Also, a comparative expression map of cell cycle-related genes expressed in the oocyte and cu-mulus cells was established using nylon-based cDNA arrays, which allowed the detection of 35 different genes transcribed mostly in oocytes. Both components of the pre-MPF complex were expressed at the mRNA level in GV oocytes, whereas p34 cdc2 was the only pre-MPF protein detected at that stage, thus indicating that meiosis resumption in bovine oocytes is differentially regulated as compared with other mammals, and mei-osis resumption seems to be regulated by the translation of cy-clin B1 mRNA.