Transcriptional up‐regulation of the VCAM‐1 gene, induced by proinflammatory cytokines such as IL‐1β and TNF‐α, requires activation of not only NF‐κB, but also involves interferon regulatory factor (IRF)‐1. During a study of gene induction by mechanical stimuli in cultured human endothelial cells, we noted that medium hyperosmolarity appeared to influence cytokine‐induced expression of VCAM‐1. Indeed, addition of hyperosmotic, pathophysiologically relevant concentrations of NaCl effectively inhibited IL‐1β or TNF‐α induction of VCAM‐1, but not E‐selectin, at the level of mRNA and cell surface protein. Because induction of both VCAM‐1 and E‐selectin by these cytokines is NF‐κB dependent, we investigated whether the inhibitory effect of hyperosmotic medium might involve IRF‐1. Electrophoretic mobility shift assays of the VCAM‐1 promoter demonstrated that hyperosmotic medium suppressed IL‐1β‐ or TNF‐α‐activated binding activities of IRF‐1, but not NF‐κB, to their respective sites. Hyperosmotic medium also inhibited the expression of IRF‐1 induced by TNF‐α or IFN‐γ. Furthermore, hyperosmotic medium inhibited TNF‐α or IFN‐γ induction of guanylate binding protein‐1, another IRF‐1‐dependent gene. Taken together, hyperosmolarity selectively inhibits cytokine‐induced VCAM‐1 in endothelial cells, via an IRF‐1‐dependent mechanism. Thus, pathophysiological fluctuations in plasma osmolarity may influence certain endothelial‐dependent components of the inflammatory response and host defense mechanisms.