The attachment of the blastocyst to the maternal endometrium, with subsequent invasion of trophoblast and the establishment of nutrient channels for the embryo, is a critical period in early pregnancy. Over the space of a few days an embryo-maternal dialogue must be established to sustain the life of the corpus luteum, which would otherwise decline at the end of the cycle. The endocrine products of the corpus luteum, principally progesterone and facilitatory oestrogen, are required to transform the endometrium, which in turn provides a variety of proteins and other substances. The precise inventory of substances and their physiological effects are as yet unknown, since it has proved difficult in primates to obtain data on the local interactions at the site of implantation and in the corpus luteum.