The process of blastocyst implantation in mammals is remarkably variable, especially in the extent of trophoblast invasion of the endometrium. In most species studied, the earliest macroscopically identifiable sign of blastocyst implantation is an increase in endometrial vascular permeability in areas adjacent to the blastocysts. This is followed in species with invasive implantation by decidualization, again localized to areas adjacent to the blastocysts. In some species, the application of a stimulus to the endometrium can result in increased endometrial vascular permeability and decidualization. Based initially on studies utilizing inhibitors of prostaglandin (PG) synthesis and more recently on studies using the techniques of transgenics, considerable evidence has accumulated indicating that PGs have an important role in the early events of implantation and artificially induced decidualization. However, which PGs are involved remains controversial. There may be differences between species, and different PGs may be involved at different times.