THE mouse blastocyst consists of a group of cells, the inner cell mass (ICM), attached to a small area of the inside surface of a sphere of trophectoderm cells. The trophecto-derm gives rise to the extraembryonic regions of the conceptus while the ICM forms the embryo proper1. At implantation trophoblast cells invade the uterine epithelium. Those overlying the ICM proliferate to form the ectoplacental cone and the extraembryonic ectoderm and the remainder undergo transformation into giant cells2. The cells of the inner cell mass divide further, and, together with the extraembryonic ectoderm, give rise to a column of cells, the egg cylinder, that extends into the blastocyst cavity (blastocoele). Lactic dehydrogenase A subunits (4A, LDH-5) appear at this time3, and may be synthesised first in tissue deriving from the ICM4. Subsequently LDH-4 (3A1B) and LDH-3 (2A2B) appear as the contribution of B subunits to the pattern increases. If implantation is prevented, further development of the blastocyst is arrested5 and LDH-5 fails to appear3. Implantation therefore seems to act as a trigger, not only for trophoblast proliferation, but also, directly or indirectly, for further development of ICM. In the work reported here we have varied the conditions of culture of isolated blastocysts and have elicited alternative forms of development that mimic those observed in vivo when implantation occurs or is prevented. We have shown further that the triggering of ICM development in vitro may represent the release from an inhibitory effect of trophectoderm.