Factors affecting the production of platelet activating factor (PAF) by mouse embryos during culture in vitro were investigated. Detectable levels of embryo-derived PAF were produced within 1-4 hr with maximum PAF activity being observed after 6 hr of culture in vitro. The amount of PAF detected in media after 24 hr of culture of two-cell embryos was equivalent to 12.8 ng PAF/embryo. However, differences in activity were apparent with increased time in culture. Reduced synthesis of PAF during culture in vitro was supported by the observation that morulae stage embryos collected fresh from the reproductive tract displayed more PAF activity than morulae resulting from the 48 hr culture of two-cell embryos. In addition to determining production characteristics of PAF by embryos, we also show that the production of CO2 from carbon-1 position of lactate is positively correlated with the ability of embryos to develop during subsequent culture in vitro and therefore could be used as a measure of embryo viability. Furthermore, culture of embryos in media supplemented with PAF resulted in an increase in lactate utilization demonstrating a direct effect of PAF on the embryo. As PAF is produced by preimplantation embryos, an autocoid role of PAF in regulating embryo development is implicated. Therefore, the reduced production of PAF by embryos in vitro may explain the decreased viability of embryos commonly observed following their culture in vitro.