Female fertility in mammals is determined by the pool of primordial follicles and low doses of radiation induce a major loss of primordial follicles in the ovary. We investigated the expression of p53 and its homologues, p63 and p73, in the normal and irradiated neonatal ovary. p63 was the only member of the p53 family detected in oocyte nucleus. No p63 transcripts or protein were detected in the early foetal ovary. p63 production began in late pachytene-stage oocytes and peaked in diplotene oocytes in mice and humans. The production of p63 was correlated with meiotic DNA double-strand break repair. Only transactivation (TA) isoforms were present in the ovary, with