SUMMARY The effects of progesterone, medroxyprogesterone acetate (MPA), and oestrone on shedding and dissolution of the zona pellucida were studied in intact pregnant rats, ovariectomized pregnant, and ovariectomized pseudopregnant rats into whose uteri blastocysts had been transferred. Treatment with progesterone, MPA and oestrone delayed shedding of the zona. The highest percentage of blastocysts delaying, and with the longest delay period, occurred in ovariectomized, progesterone-treated rats. Ovariectomy of pregnant rats, and treatment of ovariectomized rats with progesterone, MPA or oestrone, delayed dissolution of the zona. Oestrone together with progesterone caused both shedding and dissolution of the zona within 24 h. It is deduced that in the intact pregnant rat, the synergistic action of oestrogen superimposed on progesterone synchronizes shedding and dissolution of the zona so that they occur together at a specified time.