SUMMARY 1. The action of progesterone on the development of the mouse ovum from the eight-celled stage to the blastula has been studied in vitro . 2. Concentrations of progesterone below 2 μg/ml. produced no observable effect, whereas some toxicity was observed at 4 μg/ml. and few ova survived higher concentrations. 3. Herniation of the zona pellucida by the developing blastula is described; this process was inhibited by 4 μg/ml., but not by 2 μg/ml. of progesterone. 4. Blastulae appear to be more sensitive to the action of progesterone than are earlier stages. 5. Oestrogens afforded no demonstrable protection against the toxic action of progesterone when added to the culture medium. 6. The possible physiological significance of these findings is discussed.