Several studies indicate that prostaglandins and platelet activating factor (PAF) affect implantation of the embryo. Most studies relate to animal data on prostaglandin synthesis at the site of implantation and on prostaglandin production by blastocysts. The few experimental data available on human implantation suggest that prostaglandins and PAF behave similarly in man. A review of the literature on this subject suggests possible mechanisms for prostaglandin and PAF participation in the process of human implantation.