Fully grown mouse oocytes are normally competent to progress from prophase I to metaphase II without interruption. However, growing mouse oocytes initially become only partially competent to undergo meiotic maturation. Meiotic maturation in these oocytes does not progress beyond metaphase I. In contrast to the oocytes of most strains of mice, most oocytes of strain LT/Sv mice become arrested at metaphase I even when they are fully grown. The initiation of oocyte maturation is correlated with an increase in p34^cdc2 kinase activity that continues to rise until metaphase I. The transition into anaphase I is normally correlated with a decrease in p34^cdc2 kinase activity. This study demonstrated that metaphase I arrest in both partially competent growing oocytes and fully grown LT/Sv oocytes is correlated with a sustained elevation of p34^cdc2 kinase activity. In fact, p34^cdc2 activity continued to increase during the time when activity normally decreased. In normally maturing oocytes, some, but not all, of the cyclin B, the regulatory protein associated with p34^cdc2, became degraded in oocytes that entered anaphase I. In contrast, the amount of cyclin B present in the metaphase I-arrested oocytes continued to increase at the time when it was being degraded in normal oocytes progressing to metaphase II. These results suggest that the progression of meiosis is arrested at metaphase I in both groups of oocytes because of continued p34^cdc2 kinase activity sustained, at least in part, by restricted degradation of cyclin B. Finally, metaphase I arrest in these oocytes is sustained by a mechanism that differs from that maintaining metaphase II arrest, since an inhibitor of protein phosphorylation, 6-DMAP, induced a dramatic decline in p34^cdc2 kinase activity and the resumption of meiosis in metaphase I-arrested oocytes but not in metaphase IIarrested oocytes. Moreover, without 6-DMAP treatment, cyclin B was more stable in the metaphase I-arrested oocytes than in metaphase II-arrested oocytes.