In early embryo loss, the activation of maternal immune effector mechanisms play a critical role in determining the success or failure of a pregnancy. We have previously shown that increased nitric oxide production by decidual macrophages is involved in early embryo loss occurring at day 12 of gestation. In this study, using reverse transcription-PCR and Southern blotting, the expression of inducible nitric oxide synthases (iNOS) and TNF-alpha mRNA was determined to quantify macrophage activation in individual murine embryos in a model of spontaneous early embryo loss. At day 8 of gestation, 32 and 29% of embryos with no apparent pathology showed an increase in iNOS and TNF-alpha mRNA expression, respectively. This corresponds to the natural resorption rate seen in the mouse model. In addition, the percentage of embryos with increased iNOS and TNF-alpha mRNA expression was further augmented when pregnant mice were induced to abort at a higher rate. These results showed, for the first time, a correlation between increased iNOS and TNF-alpha expression and embryo resorption. The results provide evidence for the presence of activated macrophages at implantation sites before overt embryo damage occurs.