Delayed implantation in suckling mice was interrupted by litter removal at 08.00 hr. on day 5 of lactation pregnancy. A number of preimplantation changes were examined at intervals of 2, 8 and 32 hr. after litter removal. Control groups which retained their litters were examined at similar times. Litter removal was followed by significant reduction in pituitary folliclestimulating hormone (FSH) in the 2 and 8 hr. groups. The earliest uterine response to litter removal was the incorporation of [3H]uridine. This was significantly increased by 8 hr. and was about four times the control values by 32 hr. after litter removal. Increased uterine capillary permeability and the development of dye sites were not observed until 32 hr. after litter removal. At this time the number of zygotes flushed from the uterus was significantly lower than in controls, indicating that attachment of the blastocyst to the uterus had commenced. These results indicate that delay of implantation induced by suckling is associated with increased pituitary FSH. Litter removal causes prompt release of FSH and increased oestrogenic activity in the uterus (i.e. uridine incorporation) as early as 8 hr. later.