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Stimulation of endothelial cells with agents such as interferon-y (1FN-y) and bacterial lipopolysaccharide (LPS) leads to the production of nitric oxide (NO) in a calcium-independent manner via de novo synthesis of inducible nitric oxide synthase (iNOS). Many of these agents also induce apoptosis, leading to the question as to whether NO is involved in regulating this process in endothelial cells. Previous studies, using chemical NO donors and inhibitors of NOS, have produced conflicting results. To address this issue we have used anti-sense technology to specifically decrease the synthesis of iNOS protein by generating endothelial cell lines which produce anti-sense iNOS RNA when stimulated with the ecdysone analogue GS-E. Routinely 5bM GS-E was used 24 hours prior to stimulation of cells with 1pg/ml LPS and 50U/ml IFNy for 24-72 hours; this produced up to 50% reduction in NO production compared to stimulated cells in the absence of GS-E, as measured by the Greiss reaction. The amount of iNOS protein, detected by immunoblotting, upon LPS/IFNy stimulation was also reduced in cells expressing anti-sense iNOS RNA. Using these inducible anti-sense lines, the effect of decreasing iNOS production on various measures of apoptosis was investigated. The results suggest that endogenous NO produced by iNOS protects endothelial cells from apoptosis.