MicroRNA-mediated RNA interference appears to play a role in early development and differentiation processes in preimplantation embryos. However, the expression of its key effectors, including Ago2, a key component of the RNA-induced silencing complex, and GW182, a critical component of GW bodies (GWBs), has not been assessed in preimplantation embryos. To characterise the roles of Ago2 and GW182 in early embryo development, we determined their transcription and protein synthesis in mouse embryos. Transcript levels of Ago2 and GW182 increased steadily from the one-cell stage through to the blastocyst stage when data were not normalised against an internal reference. However, when normalised against the internal standard, transcript levels for both genes were highest in four-cell stage embryos and decreased steadily through to the blastocyst stage. Indirect immunocytochemistry showed that both AGO2 and GW182 proteins were expressed in each stage in the early embryo and were observed to colocalise in the morula and blastocyst stages. Specific silencing of mRNA expression by short interference (si) RNA against Ago2 or Dicer1 decreased the expression of selected apoptosis- and development-related microRNAs, but did not inhibit development up to the blastocyst stage. However, transcription levels of Oct3/4, Nanog and Sox2 were decreased in both Ago2- and Dicer1-knockdown embryos at the blastocyst stage. Furthermore, although knockdown of these genes did not change transcript levels of GW182, GW182 protein synthesis was decreased in blastocyst stage embryos. These results suggest that Ago2 and Dicer1 regulate GW182 protein expression in mouse embryos, which is linked to microRNA biogenesis and likely to be important for differentiation in the blastocyst stage.