Sheep embryos which were transferred to recipients in oestrus 3 days before or 3 days after the donors were unable to implant even when a synchronously transferred embryo was developing successfully in the same uterus. Embryos which had spent 3 days in an advanced recipient were transferred to a recipient synchronous with the original donor. Embryos first transferred on Day 3 were slightly accelerated in their development, but retained the ability to implant normally in the 2nd recipient. By contrast, embryos first transferred on Day 6 were markedly stimulated and less able to implant in the second recipient.