Nitric oxide (NO) is elevated in the synovial fluids and sera of patients with rheumatoid arthritis (RA) and is thought to be an important proinflammatory mediator in the rheumatoid synovium. To test the hypothesis that NO might modulate the apoptosis-inducing signal pathway, we investigated the effects of NO on rheumatoid synovial-cell apoptosis induced by Fas ligation with anti-Fas antibody. Pretreatment of synovial cells with the NO donor S-nitro-N-acetylpenicillamine (SNAP) prevented the Fas-mediated induction of apoptosis. The activation of caspase-3 was required to mediate Fas-induced synovial cell apoptosis. The NO donor SNAP inhibited Fas-induced caspase-3 activation in rheumatoid synovial cells. However, NO did not interrupt Fas-induced caspase-8 cleavage or subsequent cytochrome c release into the cytosol in rheumatoid synovial cells. These data indicate that NO prevents apoptosis in rheumatoid synovial cells by directly inhibiting caspase-3 activation. Thus, we propose that NO interferes with cell death signal transduction and may contribute to rheumatoid synovial cell proliferation by inhibiting induction of apoptosis.