Vascular endothelial cells can affect hematopoiesis by releasing granulocyte-macrophage colony-stimulating factor (GM-CSF). In the present study, we show that GM-CSF production in murine aorta-derived vascular endothelial cells is regulated by two cytokines:interleukin-1 (IL-1), which induces the production of GM-CSF, and interferon gamma (IFN-gamma), which downregulates this production. The GM-CSF gene is constitutively transcribed in these cells and the transcription rate of the GM-CSF gene does not change with either IL-1 alone or IL-1 plus IFN-gamma. Whereas IL-1 treatment increases the GM-CSF mRNA half-life, simultaneous treatment with IL-1 plus IFN-gamma results in a decrease in the mRNA half-life. IL-1 also enhances IL-6 mRNA accumulation in these cells, although by increasing its transcription rate. In this case, IFN-gamma does not affect IL-6 mRNA expression. These data suggest that IL-1-induced GM-CSF expression in endothelial cells is regulated at the posttranscriptional level and that IFN-gamma specifically inhibits GM-CSF expression via destabilization of the mRNA. These suppressive effects of IFN-gamma provide evidence for an additional role of IFN-gamma in regulating GM-CSF production.