Ozone exposure in the lab and environment causes airway hyperreactivity in humans and animals lasting at least three days. In guinea pigs one day post-ozone, airway hyperreactivity is mediated by eosinophils that block neuronal M2 muscarinic receptor function, thus increasing acetylcholine release from airway parasympathetic nerves. However, mechanisms of ozone-induced airway hyperreactivity change over time so that depleting eosinophils three days post-ozone makes airway hyperreactivity worse rather than better. Ozone exposure increases IL-1 in bone marrow that may contribute to acute and chronic airway hyperreactivity. To test whether IL-1 mediates ozone-induced airway hyperreactivity one and three days post-ozone. Guinea pigs were pretreated with an IL-1 receptor antagonist (anakinra, 30mg/kg ip) thirty minutes before exposure to filtered air or to ozone (2ppm, 4 hours). One or three days after exposure, airway reactivity was measured in anesthetized guinea pigs. The IL-1 receptor antagonist prevented ozone-induced airway hyperreactivity three days, but not one day, post-ozone. Ozone-induced airway hyperreactivity was vagally mediated since bronchoconstriction induced by intravenous acetylcholine was not changed by ozone. The IL-1 receptor antagonist selectively prevented ozone-induced reduction of eosinophils around nerves and prevented ozone-induced deposition of extracellular eosinophil major basic protein in airways. These data demonstrate that IL-1 mediates ozone-induced airway hyperreactivity at three days, but not one day, post-ozone. Furthermore, preventing hyperreactivity was accompanied by decreased eosinophil major basic protein deposition within the lung suggesting that IL-1 affects eosinophil activation three days post-ozone.