Implantation did not occur at the usual time in rats ovariectomized on Days 1,2,3 or 4 post coitum (p.c.), but could be induced subsequently by injections of progesterone and oestrone, begun in some instances as late as Days 12, 14, 18 and 8 p.c., respectively. Normal numbers of implantation sites were found in most animals when injections were begun on or before Day 8 p.c. Thereafter, the number of rats with implantation sites and the number of sites observed in each rat declined. The interval after ovariectomy during which blastocysts were recovered from rats not receiving hormones corresponded with the period when implantation could be induced, suggesting that blastocysts remained viable for as long as they survived.