The basic premise of this investigation was that local hormonal control of stockpiling of the base excision repair polymerase (poly) beta within oocytes of preovulatory follicles occurs as a function of cytoplasmic maturation. There was an increase in immunoreactive poly beta in sectioned oocytes of preovulatory ovine follicles during a 12-36-hour interval following the onset of prostaglandin (PG) F2alpha-induced (Day 14 of the estrous cycle) luteal regression; this response was not observed in subordinate (nonovulatory) follicles. Accumulation of poly beta in oocytes at 36 hr after PGF2alpha was negated by treatment of ewes at 12 hr with the aromatase inhibitor Arimidex or an ovulatory dose of GnRH (which, via surge gonadotropin stimulation, acutely downregulates the proestrous rise in follicular estrogen biosynthesis). Estradiol-17beta stimulated poly beta expression (transcriptional control) in oocytes of explanted (12 hr after PGF2alpha) follicles (24-hour incubation). We suggest that a critical period of estrogen amplification in the preovulatory follicle underscores the capacity of its oocyte to efficiently repair DNA and therefore reconcile spontaneous infidelities in genomic integrity that inevitably occur during preimplantation embryogenesis.