Descriptive studies have led to the conclusion that the trophoblastic giant cell transformation of the blastocyst is a component part of the implantation process in the mouse (see Snell, 1956, for review). Since the transformation can be recognized in blastocysts flushed from the uterus in both the mouse (Dickson, 1963, 1966) and the rat (Psychoyos, 1961, fig. 5), it is possible to study the effects of ovarian hormones on the blastocyst. The transformation of the mouse blastocyst can be induced, as can its implantation, after ovariectomy on the 4th day of gestation by the s.c. administration of 2·5 mg. progesterone, whereas neither transformation nor implantation follows administration of the same dose on the 6th, 7th and 8th days (Dickson, unpublished observations). The addition of a suitable dose of oestrogen to the progesterone administered at this time might be expected to induce the transformation, for 1 μg. oestradiol will induce implantation