In the rat, the blastocyst sheds its zona pellucida during the afternoon of day 5 of pregnancy (Dickmann, 1967), at which time the animal is under the influence of progesterone and oestrogen. It is not clear to what extent progesterone and oestrogen influence the process of zona shedding, although some information concerning this problem is available. It was shown that blastocysts do not shed their zonae, for at least 3 days, if transferred to the uteri of ovariectomized rats which have been treated with progesterone for several days (Dickmann & DeFeo, 1967). In addition, it has been suggested that lysis of the zona in the rat (Alloiteau & Psychoyos, 1966; Psychoyos, 1966) and the mouse (McLaren, 1967) is oestrogen-dependent. In order to explore further the effects of the ovarian hormones on the zona shedding process, the present experiments were designed to determine whether blastocysts shed their zonae in the uteri