Proliferation of small blood vessels in synovial tissues is one of the pathologic features of rheumatoid arthritis. In this study we tested the hypothesis that nitric oxide (NO) protects endothelial cells (ECs) against apoptogenic agents in vitro. Human umbilical-vein endothelial cells (HUVECs) were cultured with and without NO donor S -nitro- N -acetylpenicillamine (SNAP) and further incubated in the presence or absence of Z-leucine-leucine-leucine-aldehyde (LLL-CHO), etoposide, or C2-ceramide. After cultivation, apoptosis of HUVECs was quantified on the basis of disruption of mitochondrial transmembrane potential (DeltaPsim), activation of caspases, and the presence of hypodiploid DNA-positive cells. Treatment of HUVECs with LLL-CHO, etoposide, or C2-ceramide induced DeltaPsim, activation of caspase-3, caspase-8, and caspase-9 and the appearance of hypodiploid DNA-positive cells. NO production in HUVECs was clearly increased by SNAP. Apoptotic cell death in HUVECs induced by LLL-CHO, etoposide, and C2-ceramide was significantly suppressed by SNAP treatment. HUVECs in vitro expressed Bcl-2, Bcl-xL, and Bax; however, expression was not changed by SNAP treatment in the presence or absence of LLL-CHO, etoposide, or C2-ceramide. Although the molecule(s) responsible for the protective effects of NO remains to be identified, our data imply that NO protects HUVECs against mitochondrial perturbation caused by apoptogenic agents. These results suggest that NO promotes endothelial-cell proliferation and angiogenesis in the synovial tissues of patients with rheumatoid arthritis and that NO may be a therapeutic target for rheumatoid arthritis.