Interleukin 1 (IL-1) is one of the inflammatory cytokines, which plays a pivotal role in both host defense and homeostasis. Its signal is transduced by type I IL-1 receptor (IL-1RI). This report gives an insight into the regulatory mechanism of IL-1RI in both in vitro and in vivo. IL-1 up-regulates IL-1RI through prostaglandin E2 (PGE2) production on human fibroblasts. However, in the presence of indomethacin, IL-1 down-regulates the receptor by destabilizing IL-1 receptor mRNA. Type I and type II interferons (IFNs) up-regulate the expression of IL-1RI. This up-regulation leads to the increasing susceptibility of IL-1RI to IL-1, as the DNA binding of IL-1-induced NF-κB and the production of IL-1-induced IL-6 from the fibroblasts are augmented by pretreatment with IFNs. On the other hand, the expression of cell surface IL-1RI is inhibited by tyrosine kinase inhibitors, herbimycin and genistein, resulting in reduction of the kinase activity of IRAK (IL-1 receptor associated kinase) and IL-1-induced IL-6 production from the fibroblasts. Lipopolysaccharide (LPS) augments the expression of IL-1RI mRNA and cell surface molecule in the hepatocytes of mice in vivo, and the augmentation is mediated by the interaction of IL-1, IL-6, and of glucocorticoid (GC). When hepatocytes were pretreated with dexamethasone (Dex) and IL-6, the activation of IRAK was augmented in response to IL-1, indicating that IL-1 signaling is also up-regulated. In addition, IL-1 treatment ather combined administration of Dex and IL-6 into mice markedly increased the serum level of serum amyloid A. These data suggest that the expression of IL-1RI is regulated by inflammatory cytokines, PGE2, GC and LPS in vitro and in vivo. This study shows that the biological activity of IL-1 can be controlled by regulating the expression of IL-1RI, and therefore proposes the use of pharmaceutical drugs for the regulation of cytokine expression.