Polymorphonuclear leucocytes (polymorphs) were found in the endometrial stroma adjacent to implanting blastocysts approximately 100 h after mating. Areas of uterus not containing a blastocyst had very few polymorphs. Ovariectomized mice were treated with hormones to render them sensitive to a decidual stimulus, the intraluminal injection of arachis oil. Areas of uterus responding to the oil (as indicated by the Pontamine Sky Blue reaction) showed an influx of leucocytes into the endometrial stroma between 16 and 36 h after the application of the decidual stimulus. The polymorphs were largely neutrophils. The injection of oil into non-sensitized uteri did not result in polymorph infiltration nor did the injection of physiological saline (which is not deciduogenic) into sensitized uteri. It is concluded that during the early stages of decidualization there is attraction of polymorphs from the blood into the stroma.