Cytokine activation of gene expression can be mediated through signal transducer and activator of transcription (STAT) signaling pathways resulting in expression of target genes. Because many cytokines have important regulatory roles during early development, we wanted to ascertain whether STAT signaling was also active at this time and could therefore have important roles in mediating developmental processes. We have found that Stat1 and Stat3 mRNAs are present in both maternal and extraembryonic tissues during early postimplantation stages of murine development. Furthermore, analyses of STAT activity in E4.5–E9.5 decidual swellings by electrophoretic mobility shift assay demonstrated that Stat3 protein was active during this early developmental period. The identification of activated Stat3 demonstrates that STAT signaling functions during early postimplantation development in the mouse are likely to be important during early embryogenesis. Dev. Dyn. 208:190–198, 1997. © 1997 Wiley‐Liss, Inc.