Abstract Nitric oxide synthase (NOS) activity was enhanced in human umbilical vein endothelial cells (HUVECs) by the combined stimulation with IFN-γ plus IL-1β, TNF-α and LPS which was accompanied by cell death. DNA analysis of the NOS induced dead HUVECs showed that internucleosomal DNA fragmentation had occurred, suggesting that apoptosis was taken place. The enhanced NO production seemed to be associated with the death of HUVECs, however, both N G -methyl-L-arginine (L-NMMA) and nitro-L-arginine (N-arg), inhibitors of NOS, recovered the death of HUVECs by only 16%, suggesting that NO production was minimally involved in the cytokine induced apoptosis of HUVECs. Additional results demonstrated that both the induction of NOS activity and apoptosis in HUVECs declined with in vitro aging, i.e. declined with increasing PDLs of HUVECs, which may explain the decreased immunity during inflammation in aged people.