The molecules involved in determining meiotic competence were determined in porcine oocytes isolated from preantral and antral follicles of different sizes. Oocytes isolated from preantral follicles had a mean diameter of 78 microns, contained diffuse filamentous chromatin in the germinal vesicle and were incapable of progressing from the G2 to the M phase of the cycle even after 72 h in culture. Oocytes from early antral follicles had a mean diameter of 105 microns, showed a filamentous chromatin configuration and about half resumed meiosis but arrested at metaphase I (MI) when cultured. Oocytes from mid-antral (3-4 mm) and large antral follicles (5-6 mm) had mean oocyte diameters of 115 and 119 microns respectively, contained condensed chromatin around the nucleolus and progressed to metaphase II (MII) in 48% and 93% of instances respectively. Analysis of p34cdc2, the catalytic subunit of maturation promoting factor (MPF), by immunoblotting indicates that the inability of small (78 microns) oocytes to resume meiosis is due, at least in part, to inadequate levels of the catalytic subunit of MPF. On the other hand, the inability of intermediate-sized (105 microns) oocytes from antral follicles to complete the first meiotic division by progressing beyond MI appears not to be limited by levels of p34cdc2, which are maximal by this stage. We postulate that an inadequacy of molecules other than p34cdc2 limits progression of MI to MII; the acquisition of these molecules during the final stages of growth may be correlated with the formation of the perinucleolar chromatin rim in the germinal vesicle.