Embryos from ageing (20 to 148 weeks of age) and young (20 to 30 weeks of age) donors were transferred to ageing (52 to 221 weeks of age) and young (18 to 30 weeks of age) recipients to partition the effects of ageing oocytes and uterine environment on embryo mortality. More than 3300 two- to eight-cell embryos collected following superovulation at six 6-month intervals were transferred. The average number of ovulation points per ageing donor doe superovulated at the six intervals declined with age and repeated superovulation. The average number of ovulations for the young donors at the six intervals also differed slightly, the low number (forty-one) for the last group possibly being due to the crossbred strain used. Both embryo recovery and cleavage rates usually exceeded 80% and did not differ between young and ageing donors. The percentage of viable young developing from embryos transferred from ageing and young donors showed that the potential for embryo development had not been impaired during 3 years of ageing. The percentage of viable young developing from embryos transferred to ageing and young recipients indicated that conditions for maintaining pregnancy had been impaired in the ageing recipients. The average number of ovulations for a group of old does superovulated for the first time at 229 weeks of age was fourteen compared to sixty-two for young controls, and only 26% of the embryos transferred from the old does developed into neonates, whereas 45% of those from young donors developed normally. As the female ages, the uterine environment may become less conducive to prenatal development and the oocytes then show the effects of the ageing process directly or as a result of exposure of the oocyte or young embryo to the ageing oviduct. At laparotomy 12 days after transfer, the ageing recipients had 45·3% pre- and 14·8% postimplantation mortality. Corresponding values for young recipients were 33·5 and 16·0% respectively. Laparotomy increased embryo mortality in ageing females only. The percentage of embryos which developed to blastocysts in vitro paralleled the development in vivo of embryos in ageing and young donors. The overall sex ratio of 81·6 males/100 females resulting from transferred embryos was significantly different (P<0·05) from the expected figure.
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