The established model for the mechanism of action of aspirin is the inhibition of prostaglandin synthesis. However, this has never fully explained aspirin's repertoire of antiinflammatory properties. We found in acute pleuritis that aspirin, but not salicylate, indomethacin, or piroxicam, increased plasma nitric oxide (NO), which correlated with a reduction in inflammation. Inhibiting aspirin-elicited NO pharmacologically in this model nullified the antiinflammatory effects of aspirin. Moreover, aspirin was not antiinflammatory in either constitutive (eNOS) or inducible NO synthase (iNOS) knockout mice with IL-1beta-induced peritonitis. It transpires that aspirin generates NO through its unique ability to trigger the synthesis of 15-epi-lipoxin A(4). Aspirin and 15-epi-lipoxin A(4) were shown to inhibit leukocyte trafficking in an NO-dependent manner using intravital microscopy on IL-1beta-stimulated mouse mesentery. Not only did aspirin inhibit leukocyte-endothelial interaction in a manner similar to NO in wild-type mice but both aspirin and 15-epi-lipoxin A(4) had markedly reduced effects on leukocyte-endothelial cell adherence in eNOS- and iNOS-deficient mice compared with wild type. Collectively, these data suggest that aspirin triggers the synthesis of 15-epi-lipoxin A(4), which increases NO synthesis through eNOS and iNOS. This aspirin-elicited NO exerts antiinflammatory effects in the microcirculation by inhibiting leukocyte-endothelium interactions.