Interleukin 1 (IL-1) is one of the most potent proinflammatory cytokines possessing a wide spectrum of inflammatory, metabolic, hemopoietic, and immunologic properties. In addition, the IL-1 system has been considered relevant in regulating communication between the blastocyst and the endometrium. Interleukin 1 receptor type II (IL1R2) acts as a negative regulator for IL-1 actions and has been termed a "decoy receptor." The aim of this study was to determine the expression pattern of IL1R2 gene in mouse uterus during the early pregnancy. Both in situ hybridization and immunohistochemistry were performed to examine the spatial localization of IL1R2 expression in mouse uteri. Real-time quantitative polymerase chain reaction analyses were used to quantify Il1r2 messenger RNA (mRNA) level under in vivo and in vitro artificial decidualization. By transfecting Il1r2 gene in cultured stromal cells from day 4 pregnant mice, we detected the expression of Dtprp, a well-known marker for decidualization. Our results showed that IL1R2 gene expression was mainly localized in decidual cells close to the implanting embryo during days 5 to 8 of pregnancy. Under in vivo and in vitro artificial decidualization, Il1r2 was significantly upregulated. Dtprp mRNA expression was also upregulated by Il1r2 overexpression. Our data suggest that IL1R2 may play an important role during mouse decidualization.