The proposed research is to elucidate the physiological mechanisms involved in the swelling of the blastocyst, which is an important pre-requisite for implantation. Studies will be done on maximally expanding (rabbit) and minimally expanding types of blastocyst. The specific aims fall into three categories. Firstly, investigation will be done on the molecular mechanisms involved in ion and water transport across the trophectoderm. The work involves the development of a voltage clamp perfusion system, the study of ion fluxes, distribution of Na/K-ATPase pumps, tight junction permeability and the nature of polar lipids in the blastocyst. Secondly, studies will be done to elucidate the relation between aerobic metabolism and active transport inthe blastocyst through measurements of the redox state of NAD+ and the concentrations of adenylates in blastocysts under various experimental conditions. Thirdly, studies to determine whether exogenous agents, particularly those likely to arise in the mother, can affect transport across the trophectoderm. these involve studies on the action of estradiol-17Beta, prostaglandins and uteroglobin.