Two critical stages of mammalian oocyte regulation are prophase I arrest, which is important for sustaining the oocyte pool, and the progression through meiosis I (MI) to produce fertilizable eggs. We have found that the spindle assembly checkpoint protein BubR1 regulates both stages in mouse oocytes. We show that oocytes depleted of BubR1 cannot sustain prophase I arrest and readily undergo germinal vesicle breakdown, a marker for reentry into MI. BubR1-depleted oocytes then arrest before completing MI, marked by failure of polar body extrusion. Both meiotic defects in BubR1-depleted oocytes are due to reduced activity of the master regulator known as the anaphase-promoting complex (APC), brought about through diminished levels of the APC coactivator Cdh1.