Our purpose was to assess the role of platelet activating factor (PAF) in embryo implantation, we examined the effects of PAF and a PAF antagonist on the in vitro implantation of mouse embryos, using an in vitro embryo culture system in the absence of the endometrium.BDF1 mouse pronuclear-stage embryos were cultured until they developed to the two-cell, the four- to eight-cell, or the morula stage in the absence of PAF or its antagonist CV6209. The medium was then changed and supplemented with PAF or CV6209 at various concentrations. We also examined the reversible effects of PAF addition to the media containing the PAF antagonist.The addition of PAF to the culture from the two-cell stage significantly (P < 0.05) increased the rates of embryo implantation in vitro (control, 69.8%; 10(-10) M PAF, 90.1%; 10(-9) M PAF, 95.5%). Similarly, the addition of PAF to the cultures from the four- to eight-cell and morula stage also significantly (P < 0.05) increased their rates of implantation in vitro. In contrast, the addition of CV6209 to the culture significantly (P < 0.01) decreased the rates of embryo implantation in vitro. CV6209 also decreased the rate of blastocyst formation. The degree of inhibition by CV6209 decreased with the advancing stage of embryos. The addition of PAF to media containing CV6209 reversed the inhibition and restored the implantation rate in vitro.These results suggest that PAF may act directly on the mouse embryo and favor its implantation like an autocrine activating factor, irrespective of the presence or absence of the endometrium.