['Aspirin is known to exert cytoprotection by presently unidentified mechanisms. This study investigates the involvement of nitric oxide (NO) in antioxidant cellular protection induced by aspirin.', 'A 24-hour incubation with hydrogen peroxide markedly reduced viability of cultured endothelial cells. Preincubation with aspirin (3 to 30 micromol/L) protected endothelial cells from hydrogen peroxide-mediated toxicity and increased viability in a concentration-dependent fashion by up to 95% of control. This effect was specific in that other nonsteroidal anti-inflammatory drugs, such as salicylate or indomethacin, did not alter hydrogen peroxide toxicity. Aspirin-induced endothelial protection was abrogated in the presence of the NO scavenger PTIO (30 micromol/L) and the inhibitor of soluble guanylyl cyclase ODQ (1 micromol/L). Moreover, the l-arginine antagonist L-NMMA (25 micromol/L), but not its D-enantiomer, led to complete inhibition of aspirin-dependent cytoprotection. Correspondingly, aspirin enhanced NO synthase activity (citrulline formation) and intracellular cyclic GMP accumulation in endothelial cells. Protein expression of endothelial NO synthase remained unaffected in the presence of aspirin.'] Does aspirin protect endothelial cells from oxidant damage via the nitric oxide-cGMP pathway?