After ovulation, the fertile life of oocytes is short. In the present study, the fertile life of oocytes was studied in relation to the resumption of meiosis. Early on the day of pro-oestrus, meiotic resumption was advanced in rats by a brief infusion of LH; ovulation was induced 8 h later by Ovalyse, a GnRH analogue; rats were mated 13 h after receiving Ovalyse, that is at about the time of ovulation. Rats in which meiosis was not advanced were injected with Ovalyse and mated at various intervals after ovulation. Rats were killed 13 h after mating or on day 20 of pregnancy. In rats in which meiosis was not advanced that were mated around ovulation, fetal survival was about 90%. In rats with meiosis advanced by 8 h and mated around ovulation, only 44% of the ovulated oocytes with advanced meiosis developed into healthy fetuses; mortality before and after implantation was 37 and 19%, respectively. Rats in which meiosis was not advanced that were mated 8-9 h after ovulation had similar fetal survival and similar mortality before and after implantation. Thus ageing of the oocyte may occur either before, or after, ovulation. Preovulatory ageing is related to the resumption of meiosis.